Method for improving electric energy conversion efficiency of SST directly-hung overcharge host

Abstract

The invention provides a method for improving the electric energy conversion efficiency of an SST direct-hanging overcharge host, and relates to the technical field of electric vehicle charging, and the method comprises the following five steps: constructing a dynamic DC bus, collecting parameters of an overcharge host in real time, setting and controlling an optimal DC bus voltage reference value, switching a modulation mode, and locking an optimal duty ratio. By introducing dynamic DC bus voltage optimization and pre-stage multi-mode cooperative control, a working point can be adjusted to a state with better global loss according to real-time battery voltage and power requirements, so that the voltage conversion ratio of a post-stage non-isolated DC / DC converter is automatically constrained in a high-efficiency interval close to 1: 1, and the power efficiency of the non-isolated DC / DC converter is improved. According to the technical scheme, the conduction loss and the switching loss of the back-stage converter are remarkably reduced, meanwhile, in the process that the front-stage converter executes the optimal voltage instruction of the system, the most suitable modulation mode can be selected according to the current power level, and the circulating current loss in the front-stage converter under the light load condition is effectively restrained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electric vehicle charging, in particular to a method for improving the electric energy conversion efficiency of an SST direct-hanging super-charging host. BACKGROUND

[0002] With the rapid popularization of electric vehicles, the market demand for charging speed is increasingly urgent, and high-power super-charging technology has become an inevitable development trend. The SST (Solid State Transformer) direct-hanging super-charging host can be directly connected to the medium-voltage power grid, saving the heavy power frequency transformer, and has the advantages of high power density and fast response speed, etc. It is one of the current research directions of super-charging technology.

[0003] The SST direct-hanging super-charging host usually contains a front-stage AC / DC converter, a DC / DC converter (such as a dual active bridge DAB) providing electrical isolation, and a rear-stage non-isolated DC / DC converter (such as a Buck-Boost circuit) and other multi-stage power conversion structures. In the actual charging process, the voltage of the vehicle battery and the power output by the host will dynamically change in a very wide range (for example, the battery voltage from several hundred volts to nearly one thousand volts, and the power from tens of kilowatts to hundreds of kilowatts).

[0004] And the common SST direct-hanging super-charging host scheme mostly adopts a fixed voltage DC bus architecture, that is, the DC bus voltage output by the isolation DC / DC converter is set to a fixed value. Although this architecture is simple to control, it has a significant inherent defect: since the efficiency characteristics of the rear-stage non-isolated DC / DC converter are heavily dependent on the ratio of its input voltage (bus voltage) to output voltage (battery voltage) (i.e. voltage conversion ratio), and its highest efficiency point usually appears in the interval where the voltage conversion ratio is close to 1:1. The fixed bus voltage scheme cannot adapt to a wide range of battery voltages, resulting in the rear-stage converter working in a large step-down or step-up mode when the battery voltage deviates from the bus voltage, causing serious conduction loss and switching loss, and the efficiency drops sharply.

[0005] In addition, the front-stage isolation type DC / DC converter (such as DAB) itself also has a wide range of efficiency optimization problems. For example, under light load conditions, if single-phase shift modulation suitable for heavy load is still used, it will generate a large circulating current loss, further reducing the overall system efficiency. Therefore, the existing SST direct-hanging super-charging host scheme based on fixed DC bus voltage cannot simultaneously ensure that the front-stage and rear-stage power converters work in the high-efficiency interval during the dynamic charging process with wide voltage and wide power, resulting in a serious "dip" in the overall system efficiency curve in a wide operating condition range, making it difficult to balance the high-efficiency operation of all operating conditions. Therefore, the present application proposes a method for improving the electric energy conversion efficiency of an SST direct-hanging super-charging host to solve the problems existing in the prior art. SUMMARY

[0006] In order to solve the above problems, the present application aims to provide a method for improving the power conversion efficiency of SST direct hanging supercharged host, which can automatically maintain in a high-efficiency operating state, thereby realizing a wide range of comprehensive energy efficiency improvement far beyond the traditional scheme, and also bringing additional advantages of system reliability improvement and heat dissipation demand reduction due to reduced loss, and can solve the problems in the prior art.

[0007] In order to achieve the purpose of the present application, the present application is realized by the following technical scheme: a method for improving the power conversion efficiency of SST direct hanging supercharged host, comprising the following steps:

[0008] Step one, constructing a dynamic DC bus

[0009] A voltage-adjustable dynamic DC bus is constructed between the output end of the isolation type DC / DC converter and the input end of the non-isolation type DC / DC converter;

[0010] Step two, real-time acquisition of supercharged host parameters

[0011] The output parameters of the supercharged host are acquired in real time by the central controller, including battery voltage and output power;

[0012] Step three, setting and controlling of the optimal DC bus voltage reference value

[0013] Based on the acquired output parameters, the central controller dynamically calculates and sets an optimal DC bus voltage reference value with minimum total loss, and controls the output voltage of the isolation type DC / DC converter to be stable at the optimal DC bus voltage reference value;

[0014] Step four, switching of modulation mode

[0015] According to the size of the output power, the isolation type DC / DC converter is controlled to switch between different adjustment modes;

[0016] Step five, best duty cycle locking

[0017] The non-isolation type DC / DC converter is controlled to convert the optimal DC bus voltage reference value, charge the battery, and constrain the actual voltage conversion ratio of the non-isolation type DC / DC converter within the preset efficiency range.

[0018] Further improvement is that it further comprises the following steps:

[0019] Step six, closed-loop correction

[0020] A period is preset, and the central controller compares the average system efficiency in a period of time with the predicted value of the built-in loss model and performs adaptive slow calibration on the optimal DC bus voltage reference value in step three.

[0021] Further improvement lies in that the isolated DC / DC converter is a dual active bridge topology, and the non-isolated DC / DC converter is a boost-buck topology.

[0022] Further improvement lies in that the voltage adjustable range of the dynamic DC bus in step one is 600V-1000V.

[0023] Further improvement lies in that the specific way of dynamically calculating the optimal DC bus voltage reference value in step three is that:

[0024] S1, an optimal bus voltage query table is pre-stored in the central controller, and the table takes the real-time collected battery voltage and output power as indexes;

[0025] S2, an initial bus voltage setting value is obtained by querying the optimal bus voltage query table;

[0026] S3, the central controller performs perturbation near the initial bus voltage setting value, and adjusts the initial bus voltage setting value according to the feedback of the isolated DC / DC converter efficiency to obtain the optimal DC bus voltage reference value.

[0027] Further improvement lies in that the specific way of switching between different adjustment modes in step four is that:

[0028] When the output power is greater than the first preset threshold of the rated power, the isolated DC / DC converter adopts a single phase shift adjustment mode;

[0029] When the output power is less than or equal to the first preset threshold but greater than the second preset threshold, the isolated DC / DC converter is switched to a triple phase shift modulation mode;

[0030] When the output power is less than or equal to the second preset threshold, the isolated DC / DC converter is switched to a pulse skipping modulation mode.

[0031] Further improvement lies in that the first preset threshold is 30% of the rated power, and the second preset threshold is 5% of the rated power.

[0032] Further improvement lies in that the preset efficiency range in step five is 0.67-1.5.

[0033] The beneficial effects of the present application are: the present application introduces dynamic DC bus voltage optimization and front-stage multi-mode cooperative control, which can adjust the operating point to a more optimal global loss state according to the real-time battery voltage and power demand, so that the voltage conversion ratio of the rear-stage non-isolated DC / DC converter is automatically constrained in the high-efficiency interval close to 1:1, thereby significantly reducing the conduction loss and switching loss of the rear-stage converter. At the same time, in the process of executing the system optimal voltage instruction by the front-stage converter, the most suitable modulation mode can be selected according to the current power level, effectively suppressing the circulating current loss in the front-stage converter under light load conditions. This step and the voltage optimization step complement each other, together ensuring the global efficiency optimization from the system level to the component level, thereby solving the problem that the traditional fixed bus voltage scheme cannot balance the high efficiency under all working conditions in the wide voltage and wide power charging scene. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a step flowchart of the present application. DETAILED DESCRIPTION

[0035] In order to deepen the understanding of the present application, the present application will be further described in combination with the embodiments below, and the present embodiments are only used to explain the present application and do not constitute a limitation on the protection scope of the present application.

[0036] According to Figure 1 As shown in the figure, the present embodiment proposes a method for improving the electric energy conversion efficiency of the SST direct hanging super-charging host, and it should be noted that the SST direct hanging super-charging host includes a medium-voltage alternating current input port, an AC / DC converter, a dynamic DC bus, an isolated DC / DC converter, a non-isolated DC / DC converter and a battery interface connected in sequence.

[0037] Among them, the AC / DC converter converts medium-voltage alternating current into high-voltage direct current; the dynamic DC bus is composed of a group of supporting capacitors, and its voltage value is determined by the subsequent control strategy; the isolated DC / DC converter adopts a dual active bridge topology to realize electrical isolation and voltage adjustment; the non-isolated DC / DC converter adopts a boost-buck topology to accurately match the charging voltage and current of the battery. The core of the present application is a central controller (high-performance digital signal processor) responsible for executing the efficiency improvement method.

[0038] It includes the following steps:

[0039] Step 1, build a dynamic DC bus

[0040] A voltage-adjustable dynamic DC bus is constructed between the output end of the isolated DC / DC converter and the input end of the non-isolated DC / DC converter, and the voltage-adjustable range of the dynamic DC bus is set to 600 volts to 1000 volts. This range is sufficient to cover the high-efficiency charging requirements of 400V and 800V battery platforms. Further, the dynamic DC bus is supported by a group of high-performance thin-film capacitors, which are suitable for dynamic changes in high-power scenarios.

[0041] Step two, real-time acquisition of super-charging host parameters

[0042] The output parameters of the super-charging host are acquired in real time by the central controller, including the battery voltage and the output power. Specifically, the central controller acquires two key parameters in real time through sensor and controller area network bus communication, one is the battery voltage from the vehicle battery management system, and the other is the output power calculated by measurement. Specifically:

[0043] The battery voltage is directly obtained through real-time communication between the controller area network bus and the vehicle battery management system;

[0044] The output power is calculated in real time by deploying high-precision voltage and current sensors at the output end of the non-isolated DC / DC converter, sampling by DSP, and the calculation formula is:

[0045] Power = Voltage x Current.

[0046] Step three, setting and control of the optimal DC bus voltage reference value

[0047] Based on the acquired output parameters, the central controller dynamically calculates and sets an optimal DC bus voltage reference value with minimum total loss, and controls the output voltage of the isolated DC / DC converter to stabilize at the optimal DC bus voltage reference value. Specifically, the specific way to dynamically calculate the optimal DC bus voltage reference value is:

[0048] S1, prestore an optimal bus voltage lookup table in the central controller through offline simulation and experimental calibration, which uses the real-time acquired battery voltage and output power as the index. For the optimal bus voltage lookup table, it is not static data, but is established based on detailed device-level loss models including switching loss, conduction loss, magnetic core loss, etc., and is calibrated through a large amount of offline simulation and experimental data. It establishes the mapping relationship between (battery voltage, output power) and (system total loss minimization);

[0049] S2, the central controller collects the battery voltage and output power in real time, and obtains the initial bus voltage set value by querying the optimal bus voltage query table. For example, when the battery voltage is 400v and the output power is 300kw, the initial bus voltage set value is obtained by table lookup as 450v. This method ensures that an optimal starting point can be quickly obtained under any working condition, avoiding the delay caused by global search;

[0050] S3, the central controller performs perturbation around the initial bus voltage set value, and adjusts the initial bus voltage set value according to the feedback of the isolated DC / DC converter efficiency to obtain the optimal DC bus voltage reference value. Specifically, the central controller executes the following process at a fixed period (for example, 100 milliseconds):

[0051] On the basis of the initial bus voltage set value, first perturb a step (for example, 10v) upwards, calculate the instantaneous efficiency of the isolated DC / DC converter (dual active bridge converter) at this voltage, then return to the initial bus voltage set value, perturb 10v downwards again, and calculate the efficiency again. The logic is: if the efficiency after perturbation is higher than that before perturbation, it means that the perturbation direction is beneficial to efficiency improvement, and the initial bus voltage set value will be adjusted in this direction in the subsequent process. If the efficiency decreases, it will be adjusted in the opposite direction. After several iterations, the voltage set value will be stabilized around the point with the highest efficiency. This value is the optimal DC bus voltage reference value under the current working condition. Then, the isolated DC / DC converter is controlled to stabilize the output voltage at the optimal DC bus voltage reference value.

[0052] Step four, switching of modulation mode

[0053] According to the size of the output power, the isolated DC / DC converter is switched between different adjustment modes. Specifically:

[0054] When the output power is greater than the first preset threshold (30% of the rated power) of the rated power, the isolated DC / DC converter adopts the single phase shift adjustment mode with simple control and high heavy load efficiency. In this mode, the control variable is single, the algorithm is simple, and the on-state loss dominates at heavy load, so the SPS (single phase shift modulation) has excellent performance.

[0055] When the output power is less than the first preset threshold (30% of the rated power) but greater than the second preset threshold (5% of the rated power), the isolated DC / DC converter is switched to the triple phase shift modulation mode which can effectively suppress the light load circulating current loss. In this mode, TPS (triple phase shift modulation) can effectively reduce the circulating current of the transformer by introducing an additional degree of freedom (internal phase shift angle), thereby significantly reducing the circulating current loss at light load.

[0056] When the output power is less than or equal to a second preset threshold (5% of the rated power), the isolated DC / DC converter is switched to a pulse skipping modulation mode, which can greatly reduce standby loss. In the mode, PSM (pulse skipping modulation) greatly reduces the number of switches by skipping some switching cycles, thereby greatly reducing the switching loss in light load and standby.

[0057] Step five, optimal duty cycle locking

[0058] The non-isolated DC / DC converter converts the optimal DC bus voltage reference value, charges the battery, and constrains the actual voltage conversion ratio (battery voltage to optimal DC bus voltage reference value) of the non-isolated DC / DC converter to a preset efficiency range, i.e. 0.67-1.5. When the ratio is 1, the converter is close to the pass-through state, and the efficiency is the highest. Therefore, the rear converter can naturally work in the high efficiency area without complex control.

[0059] Step six, closed-loop correction

[0060] A period (for example, 10 minutes) is preset, so that the central controller compares the average system efficiency in a period of time with the predicted value of the built-in loss model, and performs adaptive slow calibration on the optimal DC bus voltage reference value in step three. Specifically, by monitoring the working state of the system in real time (including the current, voltage and switching frequency of each switching device), and based on the known device parameters in the circuit (including the on-resistance of the switching tube, the switching characteristic, and the magnetic core loss characteristic of the magnetic element), the total power loss of the isolated DC converter and the non-isolated DC converter at the current working point is calculated and accumulated in real time; the system predicted efficiency is obtained by dividing the output power by the sum of the output power and the total power loss; the central controller compares the average system efficiency measured in a period of time with the predicted efficiency calculated by the model, if the deviation between the two exceeds a predetermined threshold, it is determined that the actual device parameters or working environment have changed, and the reference value of the lookup table used in step three is slowly and adaptively calibrated, so that the measured efficiency approaches the predicted efficiency.

[0061] Further, on an SST direct-hanging super-charging host platform with a rated power of 600kW, a test object is an electric vehicle equipped with an 800-volt battery system, and the charging process follows a typical constant-current-constant-voltage characteristic curve. In the comparison, the bus voltage is fixed at 1500 volts as a comparative example, and the test working points are divided into the charging starting stage, the midpoint of the constant-current stage, the starting point of the constant-voltage stage, the midpoint of the constant-voltage stage, and the end of the charging. The comparison results are shown in the following table:

[0062]

[0063] As shown in the above table, at each test working point, the efficiency of the scheme of the application is higher than that of the comparative example, so the application solves the problem that the traditional scheme cannot simultaneously consider high efficiency in all working conditions under the wide voltage and wide power charging scene by the dynamic bus voltage optimization and the front-stage modulation mode cooperative control.

[0064] By introducing the dynamic DC bus voltage optimization and the front-stage multi-mode cooperative control, the application can adjust the working point to a more optimal global loss state according to the real-time battery voltage and power demand, so that the voltage conversion ratio of the rear-stage non-isolated DC / DC converter is automatically constrained in the high-efficiency interval close to 1:1, thereby significantly reducing the conduction loss and switching loss of the rear-stage converter. At the same time, in the process of executing the system optimal voltage instruction by the front-stage converter, the most suitable modulation mode can be selected according to the current power level, so as to effectively suppress the circulating loss inside the front-stage converter under the light load condition. This step and the voltage optimization step complement each other, and together guarantee the global efficiency optimization from the system level to the component level.

[0065] The basic principles, main features and advantages of the application are shown and described above. It should be understood by those skilled in the art that the application is not limited by the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the application. Without departing from the framework and scope of application of the application, various changes and improvements can be made to the application, and these changes and improvements all fall within the scope of the claimed application. The scope of protection of the application is defined by the appended claims and their equivalents.

Claims

1. A method for improving the power conversion efficiency of an SST direct-connected supercharger host, characterized in that: Includes the following steps: Step 1: Constructing a dynamic DC bus An adjustable dynamic DC bus is constructed between the output of an isolated DC / DC converter and the input of a non-isolated DC / DC converter. Step 2: Real-time collection of supercharger host parameters The output parameters of the supercharger are collected in real time by the central controller, including battery voltage and output power. Step 3: Setting and controlling the optimal DC bus voltage reference value; Based on the collected output parameters, the central controller dynamically calculates and sets an optimal DC bus voltage reference value with the minimum total loss, and then controls the output voltage of the isolated DC / DC converter to stabilize at the optimal DC bus voltage reference value. Step 4: Switching the modulation mode Depending on the output power, the isolated DC / DC converter can be switched between different adjustment modes. Step 5: Locking the Optimal Duty Cycle The non-isolated DC / DC converter is controlled to convert the optimal DC bus voltage reference value, charge the battery, and constrain the actual voltage conversion ratio of the non-isolated DC / DC converter within a preset efficiency range.

2. The method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: It also includes the following steps: Step Six: Closed-Loop Calibration A preset period is set so that the central controller compares the average system efficiency over a period of time with the predicted value of the built-in loss model, and performs adaptive slow calibration on the optimal DC bus voltage reference value in step three.

3. The method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: In step one, the isolated DC / DC converter is a dual active bridge topology, and the non-isolated DC / DC converter is a buck-boost topology.

4. The method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: In step one, the adjustable voltage range of the dynamic DC bus is 600V to 1000V.

5. A method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: In step three, the specific method for dynamically calculating the optimal DC bus voltage reference value is as follows: S1. An optimal bus voltage lookup table is pre-stored in the central controller, and the table is indexed by the real-time collected battery voltage and output power. S2. Obtain the initial bus voltage setpoint by querying the optimal bus voltage lookup table; S3. The central controller disturbs the initial bus voltage setpoint and adjusts the initial bus voltage setpoint based on the feedback of the efficiency of the isolated DC / DC converter to obtain the optimal DC bus voltage reference value.

6. The method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: In step four, the specific method for switching between different adjustment modes is as follows: When the output power is greater than the first preset threshold of the rated power, the isolated DC / DC converter adopts a single-phase shift adjustment mode. When the output power is less than or equal to the first preset threshold but greater than the second preset threshold, the isolated DC / DC converter switches to triple phase-shift modulation mode. When the output power is less than or equal to the second preset threshold, the isolated DC / DC converter switches to pulse skip modulation mode.

7. A method for improving the power conversion efficiency of an SST direct-connect supercharger host according to claim 6, characterized in that: The first preset threshold is 30% of the rated power, and the second preset threshold is 5% of the rated power.

8. A method for improving the power conversion efficiency of an SST direct-connected supercharger according to claim 1, characterized in that: In step five, the preset efficiency range is 0.67 to 1.5.

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