A fast charging system based on SiC / Si hybrid SST and control method thereof

Abstract

The present invention relates to a fast charging system based on SiC / Si hybrid SST, comprising an AC voltage source V ac , inductor L0, high voltage level, isolation level and low voltage level, the AC voltage source V ac After being connected in series with the inductor L0, it is connected in parallel with the high voltage stage, and the high voltage stage and the low voltage stage are electrically isolated through the isolation stage; a large number of SiC power devices are used in the existing fast charging system based on power electronic transformers, so that the system cost is still high. In view of the high cost of the fast charging system based on power electronic transformers, the present invention proposes a fast charging system based on SiC / Si hybrid SST and a control method thereof, wherein the high voltage stage uses a SiC / Si hybrid full-bridge three-level submodule, which can greatly reduce the number of SiC power devices used, thereby reducing the system cost.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a fast charging system based on a SiC / Si hybrid SST and a control method thereof. Background Art

[0002] With the popularization of new energy vehicles, the technology of electric vehicle fast charging system has developed rapidly. Power electronic transformers (SST) have the advantages of small size, light weight, high efficiency and low cost, which makes power electronic transformers used in electric vehicle fast charging systems. The electric vehicle fast charging system based on power electronic transformers is directly connected to the medium voltage power grid, without the need for traditional power frequency transformers, overcoming the shortcomings of traditional power frequency transformers such as large size, large no-load loss, uncontrollable voltage, low power density and inability to isolate faults. Compared with traditional Si devices, SiC devices have the advantages of high switching frequency and low switching loss. In order to improve the system power density and work efficiency, the existing fast charging system based on power electronic transformers uses a large number of SiC devices. The current price of high-power SiC devices is 6 to 8 times that of traditional Si devices. Since a large number of SiC devices are used in the existing fast charging system based on power electronic transformers, the cost of the entire system is still high. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a fast charging system based on SiC / Si hybrid SST and a control method thereof to solve the problem of high cost of electric vehicle fast charging systems based on power electronic transformers in the prior art.

[0004] In order to solve the above technical problems, the technical solution of the present invention is to provide a fast charging system based on SiC / Si hybrid SST, the innovation of which is: including an AC voltage source V ac , inductor L0, high voltage level, isolation level and low voltage level, the AC voltage source V ac After being connected in series with the inductor L0, it is connected in parallel with the high voltage stage, and the high voltage stage and the low voltage stage are electrically isolated by the isolation stage;

[0005] The high-voltage stage adopts a three-level full-bridge cascade multi-level converter structure, the three-level full-bridge cascade multi-level converter structure includes N high-voltage sub-modules, the high-voltage sub-module is a full-bridge three-level sub-module, the N full-bridge three-level sub-modules are connected in a serial-in and parallel-out manner, the full-bridge three-level sub-module includes M SiC / Si hybrid full-bridge three-level sub-modules and NM full-bridge three-level sub-modules of all-Si devices, N≥M≥1;

[0006] The isolation stage is composed of N isolation sub-modules, the isolation sub-module adopts an LLC resonant converter, the high-voltage side of the isolation sub-module adopts a series dual half-bridge structure of all-SiC devices, and the low-voltage side of the isolation sub-module adopts a full-bridge two-level structure. The number of isolation sub-modules and high-voltage sub-modules is the same, and each isolation sub-module is connected in series with each high-voltage sub-module;

[0007] The low-voltage stage includes several DC / DC converters, and the low-voltage DC port is connected to the photovoltaic power generation system, the energy storage system and the new energy vehicle through several DC / DC converters to realize power transmission.

[0008] Furthermore, the number of the full-bridge three-level sub-modules is determined by the voltage V PN And it is determined by the voltage resistance level of the power devices selected for each high-voltage sub-module.

[0009] Furthermore, the specific structure of the SiC / Si hybrid full-bridge three-level submodule is as follows: one of the bridge arms uses all Si devices, the two switch tubes in parallel with the clamp switch tube in the other bridge arm use SiC devices, and the remaining switch tubes are all Si devices.

[0010] Furthermore, in the SiC / Si hybrid full-bridge three-level submodule, the bridge arm using all-Si devices adopts a DNPC structure, and the bridge arm using a SiC / Si hybrid adopts an ANPC structure.

[0011] Furthermore, the low-voltage sides of the isolation submodules are connected in parallel.

[0012] In order to solve the above technical problems, the present invention also provides a control method for a fast charging system based on a SiC / Si hybrid SST, the innovation of which is that it is specifically divided into a high-voltage level control method S1 and an isolation level control method S2, wherein in the high-voltage level control method S1, only one SiC / Si hybrid submodule at the high-voltage level performs high-frequency operation, and the remaining high-voltage submodules perform low-frequency switching. Specifically, it includes the following steps:

[0013] S1: The specific steps of high-voltage level control method are:

[0014] (1) According to the high-voltage DC bus voltage V PN And the voltage rating of the switch device selected by the high-voltage submodule is V DS Determine the number N of high-voltage submodules;

[0015] (2) Setting the high-voltage DC bus reference voltage Collect the inductor current I on the high voltage AC side L0 , AC voltage source voltage V ac And the high voltage submodule voltage V SMN, and set the sampling time;

[0016] (3) According to the DC bus reference voltage in steps (1) and (2) And the number of high-voltage sub-modules N calculates the reference voltage of the high-voltage sub-module And the actual average voltage of the high-voltage submodule V SM , the calculation formula is

[0017] (4) The actual voltage average value V of the high-voltage submodule calculated in step (3) is SM With high voltage submodule reference voltage The voltage error signal V of the high-voltage submodule is obtained by subtracting er1 It is sent to the PI controller, and the active control feedback signal is obtained after the PI controller in

[0018] (5) The high-voltage AC side inductor current I collected in step (2) is L0 The active component of the d-axis and the reactive component of the q-axis are obtained through single-phase dq transformation, and the active component of the d-axis and the reactive component of the q-axis are used as active control feedback signals I d and reactive power control feedback signal I q ;

[0019] (6) Set the active reference signal to The reactive reference signal is The active control feedback signals obtained in step (4) and step (5) are and I d Add active reference signal after subtraction Get the error signal I of the active component der , the specific formula is: The error signal I der Send to PI controller to get control signal The reactive power control feedback signal I obtained in step (5) is q With reactive reference signal After the difference is made, the error signal I of the reactive component is obtained. qer , specifically: The error signal I qer Send to PI controller to get control signal Eliminate the coupling between active and reactive components through dq decoupling;

[0020] (7) The control signal The voltage reactive component of the high voltage AC side inductor L0 Add the control signal Vd , specifically The control signal The active voltage component of the high voltage AC side inductor L0 Subtract to get the control signal V q , specifically The control signal V d With the control signal V q The single-phase voltage reference command V1 which is generated by the dq inverse converter is a sine or cosine as a high-voltage level modulation signal;

[0021] (8) The high voltage submodule reference voltage calculated according to step (3) The voltage reference command is normalized to obtain the reference signal V2:

[0022]

[0023] (9) The step wave V is obtained by rounding the reference signal V2. step1 , and then the reference signal V2 and the step wave V step1 After subtracting, we get the step wave V step2 , and determine the modulation wave V r :

[0024]

[0025] V r =V2-V step1 -floor(V2-V step1 )

[0026] (10) Comparing the modulated wave with the triangular carrier wave to obtain a high-frequency SiC / Si hybrid full-bridge three-level sub-module driving signal;

[0027] (11) According to the step wave V step1 The value of V determines the number of high-voltage submodule capacitors that need to be put into use at each moment. step1 The absolute value of is equal to the number of submodule capacitors required;

[0028] (12) The capacitor voltage averages of the high-frequency SiC / Si hybrid full-bridge three-level submodules and the remaining high-voltage submodule capacitor voltages are sorted from low to high, and the inductor current I L0 、V step1 The high-voltage submodule capacitor voltage sorting results determine the high-voltage submodule capacitor that should be put into use. The judgment conditions are as follows:

[0029]

[0030] S2: The specific steps of the isolation level control method are:

[0031] (1) Determine the switching frequency, calculate the parameters of the resonant inductor and resonant capacitor, and set the dead time.

[0032] (2) The switch tube is turned on and off by controlling the 50% duty cycle, and the two tubes in the middle of the double half-bridge are complementary to the remaining two tubes. The low-voltage side of the isolation stage is uncontrolled rectification.

[0033] Further, according to the high-voltage DC bus voltage V PN The method for determining the number N of high-voltage submodules is as follows: under the same device withstand voltage level, the higher the DC bus voltage level, the more high-voltage submodules there are; according to the withstand voltage level V of the switch device selected by the high-voltage submodule DS The formula for determining the number of high-voltage submodules N is: DS >(V PN / 2N).

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) A large number of SiC power devices are used in the existing fast charging system based on power electronic transformers, which makes the system cost still high. In order to solve the problem of high cost of fast charging system based on power electronic transformers, the present invention proposes a fast charging system based on SiC / Si hybrid SST, in which the high voltage stage uses a SiC / Si hybrid full-bridge three-level submodule, which can greatly reduce the number of SiC power devices used, thereby reducing the system cost.

[0036] (2) With respect to the system structure adopted by the present invention, a control method for a fast charging system based on a SiC / Si hybrid SST is proposed. In the proposed control method, high-frequency switching is concentrated on the SiC device of a SiC / Si hybrid full-bridge three-level submodule, and the remaining submodule switch tubes only perform low-frequency switching. Under the premise of ensuring system performance and function, the number of SiC / Si hybrid full-bridge three-level submodules used can be further reduced, thereby reducing system cost.

[0037] (3) The existing fast charging system based on power electronic transformer uses two-level submodules in the high-voltage stage, resulting in a large number of system submodules and low power density. To address the problem of low power density, the high-voltage stage uses a full-bridge three-level submodule, whose output voltage is a multi-level waveform, so the harmonic interference is small. At the same time, it can reduce the number of submodules required for the high-voltage stage, thereby improving the power density of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a structural diagram of a fast charging system based on SiC / Si hybrid SST proposed by the present invention.

[0039] Figure 2 This is the working principle diagram of the high-voltage and high-frequency SiC / Si hybrid full-bridge three-level submodule.

[0040] Figure 3 It is a structural diagram of a full-bridge two-level sub-module, a SiC / Si hybrid full-bridge three-level sub-module, and a full-bridge three-level sub-module of all-Si devices.

[0041] Figure 4 It is a structural diagram of the existing fast charging system solution.

[0042] Figure 5 : is the reference voltage signal waveform V1 after voltage and current control in the embodiment.

[0043] Figure 6 The reference signal waveform V2 is obtained by normalizing the reference voltage signal after voltage and current control with 1 / 2 of the high-voltage submodule reference voltage as a reference in the embodiment.

[0044] Figure 7 is the waveform V obtained by rounding the normalized reference signal waveform V2 in the embodiment. step1 .

[0045] Figure 8 In the embodiment, the reference signal V2 and the step wave signal V step1 The reference signal waveform V3 is obtained after the difference.

[0046] Fig. 9 is the waveform V obtained by rounding the reference signal V3 in the embodiment. step2 .

[0047] Fig.10 The reference signal V3 and the waveform V step2 The modulation wave V of the SiC / Si hybrid full-bridge three-level submodule with high frequency operation obtained after subtraction r .

[0048] Fig.11 is the sorting result of the average value of the capacitor voltage of the high voltage submodule 1 in the embodiment.

[0049] Fig.12 is the capacitor voltage U of the high-voltage submodule 2 in the embodiment C1 The sorting results of .

[0050] Fig.13 is the capacitor voltage U of the high-voltage submodule 2 in the embodiment C2 The sorting results of .

[0051] Fig.14 is the capacitor voltage U of the high voltage submodule 3 in the embodiment C1 The sorting results of .

[0052] Fig.15 is the capacitor voltage U of the high voltage submodule 3 in the embodiment C2 The sorting results of .

[0053] Fig.16 is the bridge arm voltage waveform of the high voltage submodule 1 in the embodiment.

[0054] Fig.17 : is the bridge arm voltage waveform of the high voltage submodule 2 in the embodiment.

[0055] Fig.18 : is the bridge arm voltage waveform of the high voltage submodule 3 in the embodiment.

[0056] Fig.19 : is the bridge arm voltage waveform of the high voltage stage in the embodiment.

[0057] Fig. 20 : is the output voltage waveform of the high voltage submodule 1 in the embodiment.

[0058] Fig.21 : is the output voltage waveform of the high voltage submodule 2 in the embodiment.

[0059] Fig. 22 : is the output voltage waveform of the high voltage submodule 3 in the embodiment.

[0060] Fig.23 : is the voltage waveform of the low-voltage DC bus A in the embodiment.

[0061] Fig.24 : is the voltage waveform of the high-voltage DC bus P in the embodiment.

[0062] Fig.25 : is the voltage waveform of the high voltage stage AC voltage source in the embodiment. DETAILED DESCRIPTION

[0063] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0064] The present invention provides a fast charging system based on SiC / Si hybrid SST, the specific structure of which is as follows Figure 1 As shown, including AC voltage source V ac , inductor L0, high voltage stage, isolation stage and low voltage stage, AC voltage source V ac After being connected in series with the inductor L0 and connected in parallel with the high voltage stage, the high voltage stage and the low voltage stage are electrically isolated through the isolation stage;

[0065] The high voltage stage adopts a three-level full-bridge cascade multi-level converter structure, which includes N high voltage sub-modules, wherein the high voltage sub-module is a full-bridge three-level sub-module, and the N full-bridge three-level sub-modules are connected in a serial-in-parallel-out manner, and the full-bridge three-level sub-modules include M SiC / Si hybrid full-bridge three-level sub-modules and NM full-bridge three-level sub-modules of all-Si devices, where N≥M≥1.

[0066] The specific structure of the SiC / Si hybrid full-bridge three-level submodule is as follows: Figure 3 As shown in (a): one of the bridge arms uses all-Si devices, the two switch tubes in parallel with the clamp switch tube in the other bridge arm use SiC devices, and the remaining switch tubes are all Si devices. In the SiC / Si hybrid full-bridge three-level submodule, the bridge arm using all-Si devices adopts a DNPC structure, and the bridge arm using a SiC / Si hybrid adopts an ANPC structure.

[0067] like Figure 2 The figure shows the working principle of the SiC / Si hybrid full-bridge three-level submodule with high-frequency operation at the high-voltage level. The high-voltage level voltage reference instruction V1 is normalized to 1 / 2 of the submodule reference voltage to obtain the modulation signal V2, and V2 is rounded to obtain the step wave V stair-1 , subtract V from V2 stair-1 Modulation signal V3; V3 is converted into a step wave V by taking the integer function stair-2 , subtract V from V3 stair-2 Get the modulation wave V of the submodule ref , and the modulation wave V ref With the triangular carrier V tri Comparison generates the drive signal of switches S2 and S3. ref >V tri When S2 is turned on and S3 is turned off; when V ref <V tri When V2 is 0, S2 is turned off and S3 is turned on. The driving signals of S1, S4, S5 and S6 are generated by comparing the modulation signal V2 with 0. When V2>0, S1 and S6 are turned on, S5 and S4 are turned off. When V2≤0, S1 and S6 are turned off, S5 and S4 are turned on. According to the inductor current I L0 , V stair-1 The ranking result n of the average value of the capacitor voltage of the SiC / Si hybrid full-bridge three-level submodule with high frequency operation is compared to obtain the ranking of the switch tubes S7, S8, S9 and S 10 For the high-frequency SiC / Si hybrid full-bridge three-level submodule, at least one capacitor is in the on-state when the system is working, so the average value of the two capacitor voltages is taken for sorting, and the capacitor in the bypass state is decided whether to be put into use according to the sorting result. The principle is as follows:

[0068]

[0069] For the high-voltage submodule working at low frequency: according to the high-voltage AC side inductor current I L0 , V stair-1 The capacitor voltage U of the high-voltage submodule with low-frequency operation C1 , U C2 The sorting results are compared to obtain the switch tube drive signal, and its working principle is shown in Table 1 and Table 2 below, where n c1 , n c2 is the capacitor voltage sorting result of the high-voltage submodule, and N is the number of high-voltage submodules.

[0070] Table 1

[0071]

[0072] Table 2

[0073]

[0074] The isolation stage is composed of N isolation submodules. The isolation submodule adopts LLC resonant converter. The high-voltage side of the isolation submodule adopts a series double half-bridge structure of all-SiC devices. The low-voltage side of the isolation submodule adopts a full-bridge two-level structure. The number of isolation submodules and high-voltage submodules is the same, and each isolation submodule is connected in series with each high-voltage submodule. Among them, the low-voltage side of each isolation submodule is connected in parallel.

[0075] The low-voltage stage includes several DC / DC converters. The low-voltage DC port is connected to the photovoltaic power generation system, energy storage system and new energy vehicles through several DC / DC converters to realize power transmission.

[0076] The number of full-bridge three-level submodules of the present invention is determined by the voltage V of the high-voltage DC bus P. PN And it is determined by the voltage resistance level of the power devices selected for each high-voltage sub-module.

[0077] The present invention also provides a control method for a fast charging system based on a SiC / Si hybrid SST, which is specifically divided into a high-voltage level control method S1 and an isolation level control method S2, wherein in the high-voltage level control method S1, only one SiC / Si hybrid full-bridge three-level submodule at the high-voltage level performs high-frequency operation, and the remaining high-voltage submodules perform low-frequency switching. Specifically, the following steps are included:

[0078] S1: The specific steps of high-voltage level control method are:

[0079] (1) According to the high-voltage DC bus voltage V PN And the voltage rating of the switch device selected by the high-voltage submodule is V DSDetermine the number N of high-voltage submodules; specifically, according to the voltage V PN The method for determining the number N of high-voltage submodules is as follows: under the same device withstand voltage level, the higher the DC bus voltage level, the more high-voltage submodules there are; according to the withstand voltage level V of the switch device selected by the high-voltage submodule DS The formula for determining the number of high-voltage submodules N is: DS <(V PN / 2N).

[0080] (1) According to the high-voltage DC bus voltage V PN And the voltage rating of the switch device selected by the high-voltage submodule is V DS Determine the number N of high-voltage submodules;

[0081] (2) Setting the high-voltage DC bus reference voltage Collect the inductor current I on the high voltage AC side L0 , AC voltage source voltage V ac And the high voltage submodule voltage V SMN , and set the sampling time;

[0082] (3) According to the DC bus reference voltage in steps (1) and (2) And the number of high-voltage sub-modules N calculates the reference voltage of the high-voltage sub-module And the actual average voltage of the high-voltage submodule V SM , the calculation formula is

[0083] (4) The actual voltage average value V of the high-voltage submodule calculated in step (3) is SM With high voltage submodule reference voltage The voltage error signal V of the high-voltage submodule is obtained by subtracting er1 It is sent to the PI controller, and the active control feedback signal is obtained after the PI controller in

[0084] (5) The high-voltage AC side inductor current I collected in step (2) is L0 The active component of the d-axis and the reactive component of the q-axis are obtained through single-phase dq transformation, and the active component of the d-axis and the reactive component of the q-axis are used as active control feedback signals I d and reactive power control feedback signal I q ;

[0085] (6) Set the active reference signal to The reactive reference signal is The active control feedback signals obtained in step (4) and step (5) are and Id Add active reference signal after subtraction Get the error signal I of the active component der , the specific formula is: The error signal I der Send to PI controller to get control signal The reactive power control feedback signal I obtained in step (5) is q With reactive reference signal After the difference is made, the error signal I of the reactive component is obtained. qer , specifically: The error signal I qer Send to PI controller to get control signal Eliminate the coupling between active and reactive components through dq decoupling;

[0086] (7) The control signal The voltage reactive component of the high voltage AC side inductor L0 Add the control signal V d , specifically The control signal The active voltage component of the high voltage AC side inductor L0 Subtract to get the control signal V q , specifically The control signal V d With the control signal V q The single-phase voltage reference command V1 which is generated by the dq inverse converter is a sine or cosine as a high-voltage level modulation signal;

[0087] (8) The high voltage submodule reference voltage calculated according to step (3) The voltage reference command is normalized to obtain the reference signal V2:

[0088]

[0089] (9) The step wave V is obtained by rounding the reference signal V2. step1 , and then compare the reference signal V2 with the step wave V step1 After subtracting, we get the step wave V step2 , and determine the modulation wave V r :

[0090]

[0091] V r =V2-V step1 -floor(V2-V step1 )

[0092] (10) Comparing the modulated wave with the triangular carrier wave to obtain a high-frequency SiC / Si hybrid full-bridge three-level sub-module driving signal;

[0093] (11) According to the step wave V step1 The value of V determines the number of high-voltage submodule capacitors that need to be put into use at each moment. step1 The absolute value of is equal to the number of submodule capacitors required;

[0094] (12) The capacitor voltage averages of the high-frequency SiC / Si hybrid full-bridge three-level submodules and the remaining high-voltage submodule capacitor voltages are sorted from low to high, and the inductor current I L0 、V step1 The high-voltage submodule capacitor voltage sorting results determine the high-voltage submodule capacitor that should be put into use. The judgment conditions are as follows:

[0095]

[0096] S2: The specific steps of the isolation level control method are:

[0097] (1) Determine the switching frequency, calculate the parameters of the resonant inductor and resonant capacitor, and set the dead time.

[0098] (2) The switch tube is turned on and off by controlling the 50% duty cycle, and the two tubes in the middle of the double half-bridge are complementary to the remaining two tubes. The low-voltage side of the isolation stage is uncontrolled rectification.

[0099] In order to further describe the fast charging system and control method based on SiC / Si hybrid SST, the present invention is described below with reference to examples:

[0100] The high voltage stage in the embodiment of the present invention is a single-phase cascade multi-level converter, the high voltage AC side input voltage is 3kV / 50Hz, the high voltage stage DC bus voltage is 4.8kV, the low voltage stage DC bus voltage is 1.066kV, and there are three high voltage stage and isolation stage sub-modules, among which the high voltage sub-module 1 is a SiC / Si hybrid full-bridge three-level sub-module.

[0101] like Figure 3 The following are the topological diagrams of the SiC / Si hybrid full-bridge three-level submodule, the full-bridge three-level submodule of all-Si devices, and the full-bridge two-level submodule. Since the high-voltage stage uses a full-bridge three-level submodule, each bridge arm has two series-connected tubes to withstand the submodule voltage when it is working. Compared with the existing full-bridge two-level submodule, the full-bridge three-level submodule can withstand twice the voltage of the existing full-bridge two-level submodule. When using the same device and the same DC bus voltage, the number of the two submodules can be calculated as follows:

[0102]

[0103] Where N1 is the number of full-bridge three-level sub-modules, and N2 is the number of two-level modules. From the formula, we can know that under the same DC bus voltage and switching devices, the number of sub-modules can be reduced by half compared to the use of two-level sub-modules using full-bridge three-level sub-modules.

[0104] Therefore, compared with the existing power electronic transformer fast charging system using two-level full-bridge submodules at the high voltage stage, the high voltage submodule of the embodiment of the present invention can reduce the submodules by about 50%.

[0105] The input voltage amplitude of the high-voltage AC side of the system and the high-voltage DC bus voltage must meet the following requirements:

[0106] V PN =V ac ×m

[0107] Where V PN is the high-voltage DC bus voltage, V ac is the input voltage amplitude on the AC side. m is the high voltage stage modulation ratio.

[0108] According to the formula V PN =V ac ×m can calculate the modulation ratio of the high-voltage cascade multilevel converter in this example:

[0109]

[0110] The high-voltage submodule uses a full-bridge three-level submodule, which is connected in series and parallel, so the voltage borne by the submodule needs to meet the constraints:

[0111]

[0112] Where N is the number of submodules, V SM* is the maximum voltage constraint that the submodule can withstand, from which the selection range of the voltage constraint that the submodule can withstand can be calculated. The constraint in this example is expressed as:

[0113]

[0114] V SM It is the voltage that the submodule bears. Since the isolation stage adopts a series dual half-bridge LLC structure, and the isolation submodule is connected in series with the high-voltage submodule, the voltage on the high-voltage side of the isolation submodule is equal to the voltage of the high-voltage submodule connected in series with it, so the primary and secondary voltages of the isolation stage transformer should meet the following requirements:

[0115]

[0116] VAO is the low voltage DC bus voltage, and n* is the isolation transformer ratio. From this, the isolation transformer ratio n* can be calculated:

[0117]

[0118] As shown in Figure 4, a technical solution for an electric vehicle charging system is introduced. The high-voltage stage also adopts a full-bridge three-level sub-module. Since the high-voltage sub-module of the system adopts a full-bridge three-level structure of all-SiC power devices, the number of SiC devices used has increased significantly, resulting in the cost of the entire system is still high.

[0119] Compared to Figure 4 The technical solution given, the high-voltage level SiC / Si hybrid full-bridge three-level submodule of the fast charging system based on the power electronic transformer proposed by the present invention only uses two SiC power devices, so the system proposed by the present invention has a lower economic cost. And if the high-voltage level only uses one SiC / Si hybrid full-bridge three-level submodule, then as the number of submodules increases, the reduced economic cost will be higher.

[0120] Therefore, the embodiments of the present invention are respectively Figure 4 Compared with the given system, the number of SiC / power devices used in the high-voltage submodule can be reduced by 90%.

[0121] The high-voltage reference voltage signal V1 obtained by voltage and current control is as follows Figure 5 As shown, the modulation signal V2 obtained by standardizing it with 1 / 2 submodule voltage as reference value is as follows Figure 6 shown.

[0122] The step wave V is obtained by rounding the modulation signal V2. step1 :

[0123]

[0124] like Figure 7 Shown is V step1 waveform, and then compare the reference signal V2 with the step wave V step1 After the difference, the reference signal V3 is obtained as Figure 8 As shown, V3 is rounded to get the step wave V step2 like Fig. 9 As shown, the modulation wave V r :

[0125]

[0126] V r =V2-V step1 -floor(V2-V step1 )

[0127] like Fig.10 The modulation wave V of the SiC / Si hybrid submodule is shown. r .

[0128] The modulation wave V r By comparing with the triangular carrier, the driving signal of the high-frequency-operating SiC / Si hybrid full-bridge three-level sub-module switch tube is obtained.

[0129] according to Figure 7 The step wave V step1 Determine the number of high-voltage submodule capacitors that need to be put into use at each moment, and the value of the step wave is equal to the number of high-voltage submodule capacitors that need to be put into use;

[0130] The average capacitor voltages of the high-frequency-operated SiC / Si hybrid full-bridge three-level submodules and the capacitor voltages of the remaining high-voltage submodules are sorted from low to high, such as Figures 11 to 15 The following table shows the results of the capacitor voltage sorting of the high-voltage submodule. L0 、V step1 The high-voltage submodule capacitor voltage sorting results determine the high-voltage submodule capacitor that should be put into use. The judgment conditions are as follows:

[0131]

[0132] Determine the switching frequency of the high-side of the isolation stage, calculate the resonant inductor and resonant capacitor parameters, and set the dead time.

[0133] The switch tube is turned on and off by controlling the 50% duty cycle, and the two tubes in the middle of the double half-bridge are complementary to the remaining two tubes. The low-voltage side of the isolation stage is uncontrolled rectification.

[0134] like Figures 16 to 18 The following are the voltage waveforms of the bridge arms of high-voltage submodules 1, 2, and 3. As can be seen from the waveforms, high-voltage submodule 1 is a high-frequency action submodule, and its input voltage waveform is a high-frequency PWM wave. The input voltage waveforms of the other two submodules are square waves, and they only perform low-frequency actions. Fig.19 The figure shows the bridge arm voltage waveform of the high-voltage stage. It can be seen from the figure that it is a multi-level waveform. When the high-voltage stage works in the inverter mode, the output voltage and current harmonics of the high-voltage stage are reduced, and the harmonic interference to the grid side is reduced.

[0135] like Figures 20-22 The figure shows the output voltage waveform of the high-voltage submodule. From the waveform, it can be seen that the output voltage of the three submodules is stable at around 1600V, with fluctuations of no more than 1%. Fig.23 The low-voltage DC bus voltage is shown, which is stable at about 1066V, with fluctuations of no more than 1%. Fig.24The figure shows the system high-voltage DC bus voltage, which is stable at around 4800, with fluctuations of no more than 1%. Fig.25 It is the voltage waveform of the high voltage AC voltage source.

Claims

1. A fast charging system based on SiC / Si hybrid SST, characterized by: Including AC voltage source V ac , inductor L0, high voltage level, isolation level and low voltage level, the AC voltage source V ac After being connected in series with the inductor L0, it is connected in parallel with the high voltage stage, and the high voltage stage and the low voltage stage are electrically isolated by the isolation stage; The high-voltage stage adopts a three-level full-bridge cascade multi-level converter structure, the three-level full-bridge cascade multi-level converter structure includes N high-voltage sub-modules, the high-voltage sub-module is a full-bridge three-level sub-module, the N full-bridge three-level sub-modules are connected in a serial-in and parallel-out manner, the full-bridge three-level sub-module includes M SiC / Si hybrid full-bridge three-level sub-modules and NM full-bridge three-level sub-modules of all-Si devices, N≥M≥1; The isolation stage is composed of N isolation sub-modules, the isolation sub-module adopts an LLC resonant converter, the high-voltage side of the isolation sub-module adopts a series dual half-bridge structure of all-SiC devices, and the low-voltage side of the isolation sub-module adopts a full-bridge two-level structure. The number of isolation sub-modules and high-voltage sub-modules is the same, and each isolation sub-module is connected in series with each high-voltage sub-module; The low-voltage stage includes several DC / DC converters, and the low-voltage DC port is connected to the photovoltaic power generation system, the energy storage system and the new energy vehicle through several DC / DC converters to realize power transmission.

2. A fast charging system based on SiC / Si hybrid SST according to claim 1, characterized in that: The number of the full-bridge three-level submodules is determined by the voltage V of the high-voltage DC bus P. PN And it is determined by the voltage resistance level of the power devices selected for each high-voltage sub-module.

3. A fast charging system based on SiC / Si hybrid SST according to claim 1, characterized in that: The specific structure of the SiC / Si hybrid full-bridge three-level submodule is as follows: one of the bridge arms uses all Si devices, the two switch tubes in parallel with the clamp switch tube in the other bridge arm use SiC devices, and the remaining switch tubes are all Si devices.

4. A fast charging system based on SiC / Si hybrid SST according to claim 1, characterized in that: In the SiC / Si hybrid full-bridge three-level sub-module, the bridge arm using all-Si devices adopts a DNPC structure, and the bridge arm using a SiC / Si hybrid adopts an ANPC structure.

5. A fast charging system based on SiC / Si hybrid SST according to claim 1, characterized in that: The low voltage sides of the isolation submodules are connected in parallel.

6. A control method for a fast charging system based on a SiC / Si hybrid SST as claimed in any one of claims 1 to 5, characterized in that: Specifically, it is divided into a high-voltage level control method S1 and an isolation level control method S2. In the high-voltage level control method S1, only one SiC / Si hybrid submodule of the high-voltage level performs high-frequency operation, and the other high-voltage submodules perform low-frequency switching. Specifically, it includes the following steps: S1: The specific steps of high-voltage level control method are: (1) According to the high-voltage DC bus voltage V PN And the voltage rating of the switch device selected by the high-voltage submodule is V DS Determine the number N of high voltage submodules; (2) Setting the high-voltage DC bus reference voltage Collect the inductor current I on the high voltage AC side L0 , AC voltage source voltage V ac And the high voltage submodule voltage V SMN , and set the sampling time; (3) According to the DC bus reference voltage in steps (1) and (2) And the number of high-voltage sub-modules N calculates the reference voltage of the high-voltage sub-module And the actual average voltage of the high-voltage submodule V SM , the calculation formula is (4) The actual voltage average value V of the high-voltage submodule calculated in step (3) is SM With high voltage submodule reference voltage The voltage error signal V of the high-voltage submodule is obtained by subtracting er1 It is sent to the PI controller, and the active control feedback signal is obtained after the PI controller in (5) The high-voltage AC side inductor current I collected in step (2) is L0 The active component of the d-axis and the reactive component of the q-axis are obtained through single-phase dq transformation, and the active component of the d-axis and the reactive component of the q-axis are used as active control feedback signals I d and reactive power control feedback signal I q ; (6) Set the active reference signal to The reactive reference signal is The active control feedback signals obtained in step (4) and step (5) are and I d Add active reference signal after subtraction Get the error signal I of the active component der , the specific formula is: The error signal I der Send to PI controller to get control signal The reactive power control feedback signal I obtained in step (5) is q With reactive reference signal After the difference is made, the error signal I of the reactive component is obtained. qer , specifically: The error signal I qer Send to PI controller to get control signal Eliminate the coupling between active and reactive components through dq decoupling; (7) The control signal The voltage reactive component of the high voltage AC side inductor L0 Add the control signal V d , specifically The control signal The active voltage component of the high voltage AC side inductor L0 Subtract to get the control signal V q , specifically The control signal V d With the control signal V q The single-phase voltage reference command V1 of sine or cosine is generated by the dq inverse converter as a high-voltage level modulation signal; (8) The high voltage submodule reference voltage calculated according to step (3) The voltage reference command is normalized to obtain the reference signal V2: (9) The step wave V is obtained by rounding the reference signal V2. step1 , and then compare the reference signal V2 with the step wave V step1 After subtraction, round off to get the step wave V step2 , and determine the modulation wave V r : V r =V2-V step1 -floor(V2-V step1 ) (10) Comparing the modulated wave with the triangular carrier wave to obtain a high-frequency SiC / Si hybrid full-bridge three-level sub-module driving signal; (11) According to the step wave V step1 The value of V determines the number of high-voltage submodule capacitors that need to be put into use at each moment. step1 The absolute value of is equal to the number of submodule capacitors required; (12) The capacitor voltage averages of the high-frequency SiC / Si hybrid full-bridge three-level submodules and the remaining high-voltage submodule capacitor voltages are sorted from low to high, and the inductor current I L0 、V step1 The high-voltage submodule capacitor voltage sorting results determine the high-voltage submodule capacitor that should be put into use. The judgment conditions are as follows: S2: The specific steps of the isolation level control method are: (1) Determine the switching frequency, calculate the parameters of the resonant inductor and resonant capacitor, and set the dead time; (2) The switch tube is turned on and off by controlling the 50% duty cycle, and the two tubes in the middle of the double half-bridge are complementary to the remaining two tubes. The low-voltage side of the isolation stage is uncontrolled rectification.

7. The control method of a fast charging system based on SiC / Si hybrid SST according to claim 6, characterized in that: According to the high-voltage DC bus voltage V PN The method for determining the number N of high-voltage submodules is as follows: under the same device withstand voltage level, the higher the DC bus voltage level, the more high-voltage submodules there are; according to the withstand voltage level V of the switch device selected by the high-voltage submodule DS The formula for determining the number of high-voltage submodules N is: DS >(V PN / 2N).

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