A light load control method for a bidirectional power supply in an electric energy storage system

Abstract

The present invention proposes a light-load control method for a bidirectional power supply in an electric energy storage system, and relates to an electric energy storage system. The control method is implemented based on a 3-level Buck‑Boost hardware topology. When the bidirectional power supply is lightly loaded, the 3-level wave generation mode is switched to the 2-level wave generation mode. Otherwise, the 2-level wave generation mode is exited and restored to the 3-level wave generation mode. During the 2-level wave generation period, if the actual output voltage is higher than the expected output voltage by 99%, the PWM wave is controlled to be blocked after a normal output of a switching cycle, and the output of the PWM wave is stopped; during the blocking period, if the actual output voltage is lower than 98% of the expected output voltage, the blocking is stopped and the PWM wave is output. The technical solution proposed by the present invention reduces the switching frequency of the power tube, improves the power conversion efficiency, reduces energy loss, and increases the service life of the equipment, while ensuring that the power supply provides sufficient support for the load.

Description

Technical Field

[0001] The invention relates to an electric energy storage system, and in particular to a light-load control method for a bidirectional power supply in an electric energy storage system. Background Art

[0002] Energy storage systems can store electrical energy and release it when needed. Such systems play a very important role in power systems, renewable energy utilization, power supply and demand regulation, etc. They can overcome the temporal or local differences between energy supply and demand, and improve energy utilization by storing excess energy and releasing it during peak demand.

[0003] As a key component in energy storage systems, DC / DC bidirectional power supplies (also known as DC converters) are used in combination with various power generation equipment. They can not only convert DC power to inverters for transmission to the power grid, but also convert power from the power grid to batteries for energy storage, thus achieving efficient utilization of new energy and incorporating it into the power grid. They are widely used in power storage systems, electric vehicle charging stations, power system regulation and other fields.

[0004] The most widely used DC / DC bidirectional power supply is the bidirectional Buck-Boost converter, which can work when the input voltage is higher or lower than the output voltage to achieve bidirectional flow of energy. From the hardware topology, it can be divided into a 2-level Buck-Boost converter and a 3-level Buck-Boost converter.

[0005] The 2-level Buck-Boost converter has a simple topology, usually consisting of two switches (IGBTs), two freewheeling diodes, two inductors, and an output capacitor. When the switch is turned on, current flows from the input power supply through the switch and the inductor to store energy; when the switch is turned off, the energy stored in the inductor is released to the load and the output capacitor through the freewheeling diode. The 2-level Buck-Boost converter has a large inductor charging voltage, resulting in a large inductor current ripple. Under the same current ripple and switching frequency requirements as the 3-level converter, the 2-level Buck-Boost converter requires a larger inductor inductance, which means a larger inductor size and a larger overall size.

[0006] Due to the large current ripple and voltage stress, its efficiency and power density are not high.

[0007] The 3-level Buck-Boost converter adds two IGBT switches, two freewheeling diodes and one capacitor to the 2-level converter. This structure enables the output voltage to switch between three levels, thereby reducing current ripple and improving conversion efficiency.

[0008] The 3-level Buck-Boost converter has two additional IGBT switches, two freewheeling diodes and one capacitor. This structure can halve the pressure on a single IGBT, halve the withstand voltage when selecting an IGBT, reduce the inductor charging voltage, halve the inductance when selecting an inductor, and make the whole unit smaller.

[0009] Since the inductor charging and discharging frequency of the 3-level Buck-Boost converter is twice that of the 2-level converter, the inductor current ripple can be better suppressed.

[0010] The 3-level generator can more accurately control the output voltage and stability, achieve low harmonic distortion of input current and output voltage, improve system power factor, power utilization and system power conversion efficiency, reduce current ripple and improve system current quality and stability. It is suitable for scenarios with higher requirements on overall machine performance and appearance size.

[0011] In the traditional 3-level topology control method, the switching frequency is fixed. Since the system loss at low power and light load is mainly the power tube switching loss, the power conversion efficiency is reduced. How to set the appropriate working mode under light load, low power or no load to improve efficiency while ensuring the normal operation of the converter is a problem worth exploring. Summary of the invention

[0012] The purpose of the present invention is to improve the power conversion efficiency of a power supply under light load conditions.

[0013] To this end, on one hand, the present invention proposes a light-load control method for a bidirectional power supply in an electric energy storage system, which is implemented based on a 3-level Buck-Boost hardware topology. When the bidirectional power supply is lightly loaded, the 3-level wave generation mode is switched to the 2-level wave generation mode. Otherwise, the 2-level wave generation mode is exited and restored to the 3-level wave generation mode.

[0014] During the 2-level wave transmission period, if the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is controlled to be blocked after a normal output switching cycle, and the output of the PWM wave is stopped; during the blocking period, if the actual output voltage is lower than 98% of the expected output voltage, the blocking is stopped and the PWM wave is output.

[0015] Furthermore, the average value of the inductor current is sampled according to the actual working conditions. I L_ave , according to the inductor charging formula = Predict the inductor current ripple value Δ under the 2-level wave generation mode I L , calculate the inductor current valley value under the 2-level wave generation mode I min = I L_ave- Δ I L / 2 , when the trough value I min When ≤-2A, the bidirectional power supply is lightly loaded. I min When ≥1A, the bidirectional power supply exits the light load state; L is the inductance value of the inductor, UL is the inductor charging voltage, t on is the charging time of the inductor in one switching cycle.

[0016] Furthermore, when the bidirectional power supply is in Boost mode, UL=Uin , when the bidirectional power supply is in Buck mode, UL=Uin - Uout ,in, Uin is the input voltage, Uout is the output voltage.

[0017] Furthermore, switching to the 2-level wave generation mode is achieved in the following manner: when in Boost mode, the power tubes G2 and G3 operate completely at the same time, G1 and G4 are turned off and are only used as freewheeling diodes; when in Buck mode, the power tubes G1 and G4 operate completely at the same time, G2 and G3 are turned off and are only used as freewheeling diodes.

[0018] A possible implementation method: Use TMS320F28335 to realize control, and switch between 2-level and 3-level wave transmission modes by changing the CMP value and AQ action level.

[0019] On the other hand, the present invention further proposes a bidirectional power supply in an electric energy storage system, wherein the bidirectional power supply uses a 3-level Buck-Boost hardware topology and implements the above control method through a control module therein.

[0020] Beneficial effect: By adopting the technical solution proposed in the present invention, the 2-level wave generation mode is switched to under light load conditions, and the power tube works intermittently at the same time; the 2-level wave generation mode improves the efficiency of inductor charging, and on the premise of ensuring that the power supply provides sufficient support for the load, reduces the switching frequency of the power tube, improves the power conversion efficiency, reduces energy loss, and increases the service life of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a 3-level Buck-Boost hardware topology.

[0022] Figure 2 This is a schematic diagram of PWM with 3-level wave generation.

[0023] Figure 3This is a schematic diagram of the current flow in Boost mode when G2 and G3 are off and G1 and G4 are on.

[0024] Figure 4 This is a schematic diagram of the current flow in Boost mode when G2 is off, G3 is on, G1 is on, and G4 is off.

[0025] Figure 5 This is a schematic diagram of the current flow in Boost mode when G2 is on, G3 is off, G1 is off, and G4 is on.

[0026] Figure 6 This is a schematic diagram of the current flow in Boost mode when G2 and G3 are turned on and G1 and G4 are turned off.

[0027] Figure 7 This is a schematic diagram of the current flow in Buck mode when G1 and G4 are off and G2 and G3 are on.

[0028] Figure 8 This is a schematic diagram of the current flow in Buck mode when G1 is off, G4 is on, G2 is on, and G3 is off.

[0029] Fig. 9 This is a schematic diagram of the current flow in Buck mode when G1 is on, G4 is off, G2 is off, and G3 is on.

[0030] Fig.10 This is a schematic diagram of the current flow in Buck mode when G1 and G4 are on and G2 and G3 are off.

[0031] Fig.11 This is a schematic diagram for switching between 3-level wave transmission and 2-level wave transmission. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with the accompanying drawings. The following embodiments are only for the purpose of clearly illustrating the technical solution of the present invention and do not constitute a limitation to the present invention.

[0033] The implementation basis of the present invention is Figure 1 The 3-level Buck-Boost hardware topology shown.

[0034] Among them, the inductor stores and releases energy during the switching action: when in the inductor charging loop, the inductor stores energy; when in the inductor discharging loop, the inductor releases energy to the load and the capacitor.

[0035] It can be seen that the inductor is the main energy storage device in the entire topology. How to improve the power conversion efficiency of the inductor under light load is a problem that the present invention focuses on.

[0036] In this embodiment, the control is implemented using TMS320F28335, which is the same as the current mainstream technology implementation.

[0037] There are 6 ePWM modules in the above DSP, and each ePWM module has two outputs, ePWMxA and ePWMxB. They can be two independent single-edge PWM outputs, two independent symmetrical double-edge PWM outputs, or a pair of double-edge asymmetrical PWM outputs.

[0038] Each ePWM module contains the following sub-modules: Time-Base Module (TB), Counter Compare Module (CC), Action Qualifier Module (AQ), Dead-Band Generator Module (DB), PWM Chopper Module (PC), Trip Zone Module (TZ), Event Trigger Module (ET) and Digital Compare Module (DC).

[0039] ePWM needs a period register, a counter register, and a comparison register to generate a PWM waveform. The period register determines the size of the period, and also determines the period of PWM. The counter register counts through the clock signal. The comparison register is used to compare with the value of the period register to generate events, thereby controlling the generation of high and low levels and outputting PWM waveforms. Changing the period register can change the period of PWM, that is, the frequency of PWM. Changing the comparison register can change the duty cycle of the PWM waveform.

[0040] When the value of the counter register is equal to the value of the compare register A, a CTR=CMPA event will be generated; when the value of the counter register is equal to the value of the compare register B, a CTR=CMPB event will be generated; when the value of the counter register is equal to the value of TBPRD, a CTR=PRD event will be generated; when the value of the counter register is equal to 0, a CTR=0 event will be generated.

[0041] The generated events are sent to the AQ module to drive the output pin to produce level changes, thereby generating a PWM wave.

[0042] The TB register of the ePWM module is used to set the frequency, CMPA and CMPB are used to set the duty cycle, and AQ sets the trigger mode.

[0043] After setting TB, the counter TBCTR of the ePWM module can be increased from 0 to TBPRD and then decreased to 0, thereby obtaining a triangular carrier.

[0044] With TBCTR*pwmclk as the carrier period, the carrier is generated by increasing and decreasing counting, CMPA is used as the comparison value to generate the modulation wave, and CMPA is calculated with the switching frequency. The result is loaded into the shadow register and updated at the zero crossing point of TBCTR=ZERO. Then, the PWM wave can be generated in ePWMxA / B. The waveform of ePWMxA is as follows: Figure 2 shown.

[0045] The power tubes G2 and G3 use a carrier phase shift method and are controlled with a phase difference of 180°. The duty cycle is the same without considering the half-bus balance. The power tubes G1 and G2 are complementary tubes with complementary switching states. The power tubes G3 and G4 are complementary tubes with complementary switching states.

[0046] In the 3-level Buck-Boost mode, Vsw is the voltage of the switch node, which is a key parameter in the circuit and reflects the level state of the switch node in different working stages of the circuit.

[0047] The current flow direction of 3-level Buck-Boost in different switch combinations is as follows Figure 3-Figure 10 As shown in the figure, the arrow represents the direction of current.

[0048] In the following content, UL is the inductor charging or discharging voltage, Uin is the input voltage, Uout is the output voltage, Uc2 is the voltage of capacitor C2, Uc3 is the voltage of capacitor C3.

[0049] 1) Boost mode, duty cycle D<0.5:

[0050] Figure 3 In the middle, G2 and G3 are off, G1 and G4 are on, and inductors L1 and L2 are discharged; UL=Uin-Uout ;

[0051] Figure 4 In the middle, G2 is off, G3 is on, G1 is on, G4 is off, and inductors L1 and L2 are charged; UL=Uin-Uc2 ;

[0052] Figure 5 In the middle, G2 is on, G3 is off, G1 is off, G4 is on, and inductors L1 and L2 are charged; UL=Uin-Uc3 ;

[0053] 2) Boost mode, when duty cycle D>0.5:

[0054] Figure 6 In the middle, G2 and G3 are on, G1 and G4 are off, and inductors L1 and L2 are charged; UL=Uin ;

[0055] Figure 4 In the middle, G2 is off, G3 is on, G1 is on, G4 is off, and inductors L1 and L2 are discharged; UL=Uin-Uc2 ;

[0056] Figure 5 In the middle, G2 is on, G3 is off, G1 is off, G4 is on, and inductors L1 and L2 are discharged; UL=Uin-Uc3 ;

[0057] 3) Buck mode and duty cycle D<0.5;

[0058] Figure 7 In the middle, G1 and G4 are off, G2 and G3 are on, and inductors L1 and L2 are discharged; UL=-Uout ;

[0059] Figure 8 In the middle, G1 is off, G4 is on, G2 is on, G3 is off, and inductors L1 and L2 are charged; UL=Uc3-Uout ;

[0060] Fig. 9 In the middle, G1 is on, G4 is off, G2 is off, G3 is on, and inductors L1 and L2 are charged; UL=Uc2-Uout ;

[0061] 4) Buck mode and duty cycle D>0.5:

[0062] Fig.10 In the middle, G1 and G4 are on, G2 and G3 are off, and inductors L1 and L2 are charged; UL=Uin-Uout ;

[0063] Figure 8 In the middle, G1 is off, G4 is on, G2 is on, G3 is off, and inductors L1 and L2 are discharged; UL=Uc3-Uout ;

[0064] Fig. 9 In the middle, G1 is on, G4 is off, G2 is off, G3 is on, and inductors L1 and L2 are discharged; UL=Uc2-Uout ;

[0065] When the circuit is working, the working mode, that is, the state transition, depends on the duty cycle D.

[0066] In Buck mode, G1 and G4 are controlled. Duty cycle D refers to the duty cycle of G1 and G4. When D<0.5, G1 and G4 will not be turned on at the same time. When D>0.5, G1 and G4 will not be turned off at the same time.

[0067] In Boost mode, G2 and G3 are controlled. Duty cycle D refers to the duty cycle of G2 and G3. When D<0.5, G2 and G3 will not be turned on at the same time. When D>0.5, G2 and G3 will not be turned off at the same time.

[0068] Regardless of the situation, when D<0.5, the controlled switch tubes are not in the on state at the same time, and when D>0.5, the controlled switch tubes are not in the off state at the same time.

[0069] By setting the value of the register in the DSP, the generation of the PWM waveform is controlled, and then the on and off of each power tube is controlled by the PWM waveform to realize circuit control.

[0070] The state of the inductor and the charge / discharge voltage under different conditions are summarized as follows:

[0071]

[0072] 3-level Boost mode, when the duty cycle D<0.5, the inductor charging voltage is UL=Uin-Uc2 or UL=Uin-Uc3 , the discharge voltage is UL=Uin-Uout ; When D>0.5, the inductor charging voltage is UL=Uin , the discharge voltage is UL=Uin-Uc2 , UL= Uin-Uc3 .

[0073] 3-level Buck mode, when duty cycle D<0.5, the inductor charging voltage is UL=Uc2-Uout or UL=Uc3-Uout , the discharge voltage is UL=-Uout ; When D>0.5, the inductor charging voltage is UL=Uin-Uout , the discharge voltage is UL=Uc2-Uout or UL= Uc3-Uout .

[0074] The above is the normal control process of 3-level Buck-Boost, and the wave transmission mode is 3-level.

[0075] In this embodiment, the wave transmission mode is switched according to the current load of the power supply: when the bidirectional power supply is lightly loaded, the 3-level wave transmission mode is switched to the 2-level wave transmission mode; otherwise, the 2-level wave transmission mode is exited and restored to the 3-level wave transmission mode.

[0076] During the 2-level wave transmission period, if the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is controlled to be blocked and the PWM wave output is stopped after a normal switching cycle; during the blocking period, if the actual output voltage is lower than 98% of the expected output voltage, the blocking is stopped and the PWM wave is output.

[0077] When the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is stopped from being output, and the operating frequency of the switching tube is reduced. During the PWM period, the inductor is not charged, and the electric energy stored in it provides power output for the load. When the actual output voltage is lower than 98% of the expected output voltage, in order to ensure that the bidirectional power supply provides a normal working environment for the load, the PWM wave is stopped, the switching tube is operated, and the inductor is charged.

[0078] During the blocking period, G2 and G3 are in the off state and the inductor is discharged.

[0079] In this embodiment, the load condition of the bidirectional power supply is determined by: sampling the average value of the inductor current according to the actual working condition I L_ave , according to the inductor charging formula = Predict the inductor current ripple value Δ under the 2-level wave generation mode I L , calculate the inductor current valley value under the 2-level wave generation mode I min = I L_ave - Δ I L / 2 , when the trough value I min ≤-2A, the bidirectional power supply is judged to be lightly loaded. I min When ≥1A, the bidirectional power supply is judged to have exited the light load state.

[0080] Average inductor current I L_ave is the average value of the inductor current in one switching cycle.

[0081] in, L is the inductance value of the inductor, UL is the inductor charging voltage, t on is the charging time of the inductor in one switching cycle, which is related to the duty cycle. When the duty cycle is D, the corresponding charging time is t on =(TBPRD*D / TBPRD)*switching period=(CMP / TBPRD)*switching period.

[0082] The 3-level Buck-Boost hardware topology can be either Boost mode or Buck mode depending on the voltage at both ends. In both circuit modes, the inductor charging voltage UL different.

[0083] Determine the inductor charging voltage in the inductor charging formula based on the current circuit mode UL : When the bidirectional power supply is in Boost mode, UL=Uin , when the bidirectional power supply is in Buck mode, UL=Uin-Uout ,in, Uin is the input voltage, Uout is the output voltage.

[0084] Usually, when designing the circuit hardware parameters, the rated operating conditions of the system are to work in the CCM inductor current continuous working mode (the minimum inductor current is greater than 0). In CCM, it has the advantages of high output voltage stability, strong load resistance, and small output ripple. However, in the case of wide load and wide voltage operating range, when the load current is small, the inductor will enter the DCM discontinuous working mode (the minimum inductor current is less than 0). In the discontinuous working mode, it has the advantages of small switching loss and high efficiency.

[0085] When switching from level 3 to level 2, it is necessary to ensure that after the switching, the inductor operates in the DCM discontinuous working mode, so as to reduce the reverse recovery loss of the freewheeling diode and increase the single charging energy of the inductor.

[0086] Switching to the 2-level wave generation mode is achieved in the following way: when it is in Boost mode, the power tubes G2 and G3 act completely at the same time, G1 and G4 are turned off and only used as freewheeling diodes; when it is in Buck mode, the power tubes G1 and G4 act completely at the same time, G2 and G3 are turned off and only used as freewheeling diodes.

[0087] After the power tube is turned off, the diode connected in parallel with it is a freewheeling diode.

[0088] When the 2-level wave is generated, the power tubes G2 and G3 act at the same time in the Boost mode, and the power tubes G1 and G4 act at the same time in the Buck mode. Therefore, there is only Figure 3 , Figure 6 , Figure 7 , Fig.10 Status shown.

[0089] In Boost mode, the inductor charging voltage is UL=Uin ,like Figure 6 As shown, the discharge voltage is UL=Uin-Uout ,like Figure 3 shown.

[0090] In Buck mode, the inductor charging voltage is UL=Uin-Uout ,like Fig.10 As shown, the discharge voltage is UL=-Uout ,like Figure 7 shown.

[0091] From the above analysis, we can know that:

[0092] 3-level boost mode, when the duty cycle D>0.5, the inductor charging time is carrier period*(2*CMP-TBPRD ) / TBPRD. When D is close to 0.5, CMP is close to TBPRD / 2, and the inductor charging time is ≈0. When D<0.5, the inductor charging voltage is UL=Uin-Uc2 or UL=Uin-Uc3 ,when Uin near Uout / 2 When the inductor charging voltage UL ≈0 (under ideal conditions, Uc3, Uc2 and Uout / 2 equal).

[0093] 3-level Buck mode, when D>0.5, the inductor charging time is carrier period*(2*CMP-TBPRD ) / / TBPRD, when D is close to 0.5, CMP is close to TBPRD / 2, the inductor charging time ≈ 0, when D<0.5, the inductor charging voltage is UL= Uc2-Uout or UL=Uc3-Uout ,when Uin / 2 near Uout hour, UL ≈0 (under ideal conditions, Uc3 , Uc2 and Uin / 2 equal).

[0094] 2-level boost mode, the inductor charging voltage is UL=Uin , 2-level Buck mode, the inductor charging voltage is UL=Uin-Uout .

[0095] It can be seen that the ability to charge the inductor using the 3-level wave generation method is weaker than that using the 2-level wave generation method. When the duty cycle D is close to 0.5, the difference in the ability to charge the inductor is more obvious: for example, in Buck mode, D<0.5, Uin / 2>Uout , Uin / 2 near Uout If Uin =600, Uout =280, inductor charging voltage when 3-level wave is transmitted UL=Uc2-Uout =20V, inductor charging voltage when 2-level wave is transmitted UL=Uin-Uout =320V, the voltage when the inductor is charged is 16 times that of the 3-level wave, and the charging power is very high.

[0096] When the 3-level wave is used, the 4 states of the 2-level wave are also included. By switching the 3-level wave to the 2-level wave, the various states of the circuit are simplified to 4 states with high inductor charging efficiency, thereby improving the efficiency of inductor charging.

[0097] In actual use, when the DC / DC in the system is used in a light load and low power state, the system efficiency is low when using 3-level, and the switching loss is the main loss. Switching from 3-level to 2-level wave transmission mode, with wave transmission and wave blocking performed intermittently, can improve system efficiency and reduce losses by reducing the switching frequency.

[0098] During the 2-level wave transmission, if the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is controlled to be blocked and stop outputting the PWM wave after a normal output of a switching cycle; this method reduces the number of power tube switches while ensuring the normal operation of the power supply by improving the inductor charging efficiency.

[0099] The power conversion efficiency is the ratio of output power to input power. Under light load conditions, the switching action of the power tube is the main reason affecting the efficiency. Comprehensive tests of various working conditions show that under light load conditions, the power conversion efficiency is generally increased from less than 95% to more than 95% by using the method proposed in the present invention.

[0100] After switching to the 2-level wave mode, the number of blocking times is related to the terminal voltage, duty cycle, load current, switching frequency, etc. In Buck mode, when working at a low-voltage side voltage of 450V, a high-voltage side voltage of 1000V, a rated current of 2A, a 3-level wave switching frequency of 50kHz, and an output current of 0.4A, the duty cycle is 0.55, the actual output power is 180W, the rated output power is 900W, the IGBT switching loss is about 13W, the freewheeling diode reverse recovery loss is 5.8W, and the remaining loss is about 2.3W. In the 3-level wave mode, the power conversion efficiency is about 100%*180 / (180+13+5.8+2.3)=89.5%.

[0101] By using the method proposed in the present invention, after the PWM wave outputs one switching cycle, the wave is sealed for at least 5.5 switching cycles, and the switching frequency is 1 / 6.5 of the original one.

[0102] According to the calculation of PWM wave output for 1 switching cycle and 5.5 cycles of closed wave, the IGBT switching loss is about 13W, the freewheeling diode reverse recovery loss is 5.8W, and the remaining losses are about 17.1W. The average power conversion efficiency of 6.5 switching cycles is about 100%*180*6.5 / (180*6.5+13+5.8+17.1)=97%.

[0103] The traditional 3-level wave transmission method uses a carrier phase shift of 180 degrees, such as Figure 2 shown.

[0104] The phase shifting mode is not suitable for changing during operation. In the present invention, the carrier phase is not changed, and the power tubes G2 and G3 act completely simultaneously to achieve 2-level wave generation.

[0105] When TMS320F28335 is used for specific implementation, the 2-level and 3-level wave transmission modes can be quickly switched by changing the CMP value and the AQ action level mode.

[0106] See also Fig.11 , when the 3-level wave is emitted, the comparison value of the G2 tube is the CMPA value, and the comparison value of the G3 tube is TBPRD-CMPA; G2 tube AQ module: when TBCTR=CMPA and the count is increased, the AQ module is set low, when TBCTR=CMPA and the count is decreased, the AQ module is set high; G3 tube AQ module: when TBCTR=CMPA and the count is increased, the AQ module is set high, when TBCTR=CMPA and the count is decreased, the AQ module is set low.

[0107] When the 3-level is switched to the 2-level wave mode, the comparison values ​​of G2 and G3 are both CMPA values, the AQ action mode of G2 remains unchanged, and the AQ module of G3: when TBCTR = CMPA and the count is increased, the AQ module changes from high to low, and when TBCTR = CMPA and the count is decreased, the AQ module changes from low to high. The change of the wave mode takes effect at the zero crossing point of the carrier in the next cycle. Fig.11 The 3-level switching shown completes the switching at 2 levels at the moment.

[0108] When the 2-level is switched to the 3-level wave mode, the comparison value of G2 tube is CMPA value, the comparison value of G3 tube is TBPRD-CMPA, the AQ action mode of G2 tube remains unchanged, and the AQ module of G3 tube: when TBCTR=CMPA and counts up, the AQ module changes from low to high, and when TBCTR=CMPA and counts down, the AQ module changes from high to low. The change of the wave mode takes effect at the zero crossing point of the carrier in the next cycle. Fig.11 The 2-level switching shown completes the switching at the 3-level instant.

[0109] When a 3-level topology circuit is used for 2-level wave generation, it is possible that the positive and negative bus voltages are unbalanced. When this situation is detected, this embodiment proposes the following solution:

[0110] When the 2-level wave mode is in Boost mode, Uc2 > Uc3 When , the power tube G2 increases the conduction time, charges C3, and increases the negative bus voltage; when Uc2 <Uc3 When the power tube G3 increases the conduction time, charges C2, and increases the positive bus voltage; when the 2-level wave mode is working in Buck mode, when Uc2 > Uc3 When , the power tube G3 increases the conduction time, charges C3, and increases the negative bus voltage; when Uc2 <Uc3 When , the power tube G2 increases the conduction time, charges C2, and increases the positive bus voltage;

[0111] When ABS(V 正 -V 负 ) / >When 5V, it switches to 3-level wave transmission mode.

[0112] The present invention also proposes a bidirectional power supply in an electric energy storage system, wherein the bidirectional power supply uses a 3-level Buck-Boost hardware topology and implements the above control method through a control module therein.

[0113] The control module determines the load of the bidirectional power supply. When the bidirectional power supply is lightly loaded, the 3-level wave transmission mode is switched to the 2-level wave transmission mode. Otherwise, the 2-level wave transmission mode is exited and restored to the 3-level wave transmission mode.

[0114] During the 2-level wave transmission period, if the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is controlled to be blocked after a normal output switching cycle, and the output of the PWM wave is stopped; during the blocking period, if the actual output voltage is lower than 98% of the expected output voltage, the blocking is stopped and the PWM wave is output.

[0115] The control module samples the average value of the inductor current according to the actual working conditions I L_ave , according to the inductor charging formula = Predict the inductor current ripple value Δ under the 2-level wave generation mode I L , calculate the inductor current valley value under the 2-level wave generation mode I min = I L_ave - Δ I L / 2 , when the trough value I min When ≤-2A, the bidirectional power supply is lightly loaded. I min When ≥1A, the bidirectional power supply exits the light load state; L is the inductance value of the inductor, UL is the inductor charging voltage, t on is the charging time of the inductor in one switching cycle.

[0116] When the bidirectional power supply is in Boost mode, UL=Uin , when the bidirectional power supply is in Buck mode, UL= Uin -Uout ,in, Uin is the input voltage, Uout is the output voltage.

[0117] Switching to the 2-level wave generation mode is achieved in the following way: when it is in Boost mode, the power tubes G2 and G3 act completely at the same time, G1 and G4 are turned off and only used as freewheeling diodes; when it is in Buck mode, the power tubes G1 and G4 act completely at the same time, G2 and G3 are turned off and only used as freewheeling diodes.

[0118] The control module uses TMS320F28335 to realize control, and the switching between 2-level and 3-level wave transmission modes is realized by changing the CMP value and the AQ action level mode.

Claims

1. A light-load control method for a bidirectional power supply in an electric energy storage system, based on a 3-level Buck-Boost hardware topology, characterized in that: When the bidirectional power supply is lightly loaded, the 3-level wave transmission mode is switched to the 2-level wave transmission mode. Otherwise, the 2-level wave transmission mode is exited and restored to the 3-level wave transmission mode. The switching to the 2-level wave generation mode is achieved in the following manner: when in the Boost mode, the power tubes G2 and G3 act completely at the same time, G1 and G4 are turned off and used only as freewheeling diodes; when in the Buck mode, the power tubes G1 and G4 act completely at the same time, G2 and G3 are turned off and used only as freewheeling diodes; During the 2-level wave transmission period, if the actual output voltage is higher than 99% of the expected output voltage, the PWM wave is controlled to be blocked after a normal output switching cycle, and the output of the PWM wave is stopped; during the blocking period, if the actual output voltage is lower than 98% of the expected output voltage, the blocking is stopped and the PWM wave is output.

2. The control method according to claim 1, characterized in that: Sample the average value of the inductor current according to the actual working conditions I L_ave , according to the inductor charging formula = Predict the inductor current ripple value Δ under the 2-level wave generation mode I L , calculate the inductor current valley value under the 2-level wave generation mode I min = I L_ave - Δ I L / 2 , when the trough value I min When ≤-2A, the bidirectional power supply is lightly loaded. I min When ≥1A, the bidirectional power supply exits the light load state; in, L is the inductance value of the inductor, UL is the inductor charging voltage, t on is the charging time of the inductor in one switching cycle.

3. The control method according to claim 2, characterized in that: When the bidirectional power supply is in Boost mode, UL=Uin , when the bidirectional power supply is in Buck mode, UL=Uin-Uout ,in, Uin is the input voltage, Uout is the output voltage.

4. The control method according to claim 1, characterized in that: TMS320F28335 is used to realize control, and the switching between 2-level and 3-level wave transmission modes is realized by changing the CMP value and the AQ action level mode.

5. A bidirectional power supply in an electric energy storage system, characterized in that: The bidirectional power supply uses a 3-level Buck-Boost hardware topology, and implements the control method described in any one of claims 1 to 4 through the control module therein.

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