Cascaded Multilevel Energy Storage System and its Grid-Connected Control Method

Abstract

This application discloses a cascaded multilevel energy storage system and its grid-connected control method. The control method of the cascaded multilevel energy storage system includes: performing inter-phase equalization control of the supporting capacitor voltage on the original value of the three-phase wave emission target voltage to generate the final value of the three-phase wave emission target voltage; and, based on the final value of the three-phase wave emission target voltage and the direction of the instantaneous power of each phase, sequentially allocating the drive signals of the H-bridge unit according to the result of sorting the N power modules of each phase arm according to the magnitude of the supporting capacitor voltage value; controlling the DC-DC converter in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit, and stabilizing the voltage of the energy storage unit at the set value of the energy storage unit target voltage. This application can achieve equalization control of the supporting capacitor voltage during the grid-connected charging process, as well as the consistency of the total charging power of the three-phase energy storage units and the charging power between each energy storage unit in each phase, effectively taking into account the equalization control of the supporting capacitor voltage and the energy storage unit voltage.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a cascaded multilevel energy storage system and its grid-connected control method. Background Technology

[0002] With the increasing maturity of high-power power electronics technology, the demand for high-voltage, large-capacity energy storage systems is becoming increasingly urgent. Cascaded H-bridge multilevel converters, due to their simple main circuit topology, frequency doubling effect, good fault tolerance, and low voltage withstand requirements for components, have become the preferred solution for large-capacity energy storage systems and are widely used in high-voltage, high-power applications.

[0003] The grid-connected charging process of energy storage systems generally adopts constant current or constant power charging modes. However, due to certain differences between power units, such as differences in the initial voltage of the energy storage system, an imbalance between the supporting capacitor voltage and the energy storage unit voltage can occur during charging. If left unaddressed, the voltage deviation will continue to increase, and the difference in charging power between power units will also increase, eventually leading to overvoltage or undervoltage faults in the supporting capacitor voltage. Therefore, ensuring the balance of supporting capacitor voltage and energy storage unit voltage between the three phases and between each power unit in each phase, as well as the consistency of charging power between the three phases, is an urgent problem to be solved. Summary of the Invention

[0004] This application aims to provide a cascaded multilevel energy storage system and its grid-connected control method to solve the problem that existing cascaded multilevel energy storage systems are difficult to simultaneously achieve balanced control of the supporting capacitor voltage and the energy storage unit voltage during grid-connected operation.

[0005] This application provides a grid-connected control method for a cascaded multilevel energy storage system. The cascaded multilevel energy storage system includes an energy storage converter, which comprises three-phase bridge arms. Each phase bridge arm includes N cascaded power modules. Each power module includes an H-bridge unit, a supporting capacitor, a DC-DC converter, and an energy storage unit. The control method includes:

[0006] The original value of the three-phase wave generation target voltage is subjected to phase-to-phase equalization control of the supporting capacitor voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, the drive signals of the H-bridge unit are sequentially allocated according to the result of sorting the N power modules of each phase bridge arm by the magnitude of the supporting capacitor voltage value.

[0007] The DC-DC converters in each power module are controlled to obtain energy from the supporting capacitor to charge the energy storage unit with constant current, and the voltage of the energy storage unit is stabilized at the target voltage setting value of the energy storage unit.

[0008] Another aspect of this application provides a cascaded multilevel energy storage system, which includes an energy storage converter and a controller;

[0009] The energy storage converter includes a three-phase bridge arm, each phase bridge arm includes N cascaded power modules, each power module includes an H-bridge unit, a supporting capacitor, a DC-DC converter and an energy storage unit;

[0010] The controller is configured to perform phase-to-phase equalization control of the supporting capacitor voltage on the original value of the three-phase wave generation target voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, the controller sequentially distributes the drive signals of the H-bridge unit according to the result of sorting the N power modules of each phase arm by the magnitude of the supporting capacitor voltage value. The controller controls the DC-DC converter in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the set value of the energy storage unit target voltage.

[0011] The cascaded multilevel energy storage system and its grid-connected control method provided in this application can achieve balanced control of the supporting capacitor voltage during the grid-connected charging process, as well as consistency between the total charging power of the three-phase energy storage units and the charging power between each energy storage unit in each phase, effectively taking into account the balanced control of the supporting capacitor voltage and the energy storage unit voltage. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a cascaded multilevel energy storage system provided in an embodiment of this application;

[0013] Figure 2 This is a schematic diagram of the power module in the cascaded multilevel energy storage converter provided in the embodiments of this application;

[0014] Figure 3 This is a block diagram of the grid-connected charging process control of the cascaded multilevel energy storage system provided in the embodiments of this application;

[0015] Figure 4 This is a block diagram of grid-connected rectification control for a cascaded H-bridge topology provided in an embodiment of this application;

[0016] Figure 5 This is a block diagram of the phase-to-phase equalization control of the supporting capacitor voltage provided in the embodiments of this application;

[0017] Figure 6 This is a control block diagram of a DC-DC converter provided in an embodiment of this application;

[0018] Figure 7 This is a block diagram of phase charging current processing provided in an embodiment of this application;

[0019] Figure 8 This is a block diagram of the unit charging current processing provided in the embodiments of this application;

[0020] Figure 9 This is a schematic diagram of the grid connection control method for a cascaded multilevel energy storage system provided in the embodiments of this application.

[0021] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application.

[0023] In the description of this application, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," and "right," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0024] The variables and their definitions involved in the embodiments of this application are as follows:

[0025] i a i b i c Three-phase currents of the power grid (A, B, and C phases)

[0026] V ga V gb V gc Three-phase voltages of the power grid (A, B, and C phases)

[0027] PLL: Phase Locked Loop

[0028] θ: Phase-locked loop angle of the grid voltage, generated by the phase-locked loop (PLL).

[0029] ω: Angular frequency of the grid voltage, generated by a phase-locked loop (PLL).

[0030] V gd V gq d-axis and q-axis components of grid voltage

[0031] PI: Proportional-Integral Controller (PI Controller)

[0032] Converter active and reactive current setpoints

[0033] i d i q Feedback values ​​of active and reactive current of the converter

[0034] Support capacitor voltage setting value

[0035] U dc_abc The total average value of the three-phase voltages of the supporting capacitors A, B, and C.

[0036] U dc_a U dc_b U dc_c The average values ​​of the supporting capacitor voltages A, B, and C are as follows:

[0037] Ls: Inductance value of grid-connected PFC inductor

[0038] V d V q : d-axis and q-axis components of the converter waveform

[0039] V a V b V c Original values ​​of the target voltage for the three-phase waveform generation of converter A, B, and C.

[0040] Final values ​​of the target voltage for the three phases A, B, and C of the converter

[0041] U stor_abc Average total voltage of the three phases of energy storage units A, B, and C

[0042] U stor_a U stor_b U stor_c Average voltage of each phase of energy storage unit A, B, and C

[0043] k a k b k c Scaling factor of charging current for each phase of energy storage unit A, B, and C

[0044] k a1 k a2 ..., k aN : Scaling factor for charging current of A-phase N power module energy storage units

[0045] Unified setting value for charging current of energy storage unit (DC-DC converter)

[0046] A, B, and C phase energy storage unit charging current (DC-DC converter) setpoints

[0047] The charging current (DC-DC converter) setting value of the first to Nth stage energy storage units of phase A

[0048] u ref Target voltage setpoint for energy storage unit

[0049] u fb Energy storage unit voltage feedback value

[0050] U dc Power unit supports capacitor voltage

[0051] i L_ref Energy storage unit charging current setting value

[0052] i L Energy storage unit charging current feedback value

[0053] u pwm DC-DC converter modulation signal

[0054] Figure 1 This is a schematic diagram of a cascaded multilevel energy storage system provided in an embodiment of this application.

[0055] like Figure 1 As shown, the cascaded multilevel energy storage system includes a cascaded multilevel energy storage converter, which includes three-phase bridge arms (CELLA1 to CELLA1 in the figure). n CELLB1~CELLB n CELLC1~CELLC n As shown in the diagram, each phase arm consists of N cascaded power modules. The cascaded multilevel energy storage system also includes a grid-connected switch K1, a pre-charge resistor R, a pre-charge bypass switch K2, and a grid-connected transformer T. The pre-charge resistor R and the pre-charge bypass switch K2 are connected in parallel. The grid-connected switch K1 is connected between the power grid and one end of the pre-charge resistor R, and the grid-connected transformer T is connected between the other end of the pre-charge resistor R and the energy storage converter.

[0056] When the system is connected to the grid, the H-bridge unit in the power module operates in rectification mode, absorbing energy from the grid through the three-phase bridge arm to charge the energy storage unit in each power module; when the system is off-grid, the H-bridge unit in the power module operates in inverter mode, and the energy storage unit in the power module supplies power to the load (e.g., a motor).

[0057] Figure 2 This is a schematic diagram of the power module provided in an embodiment of this application.

[0058] like Figure 2As shown, the power module in the cascaded multilevel energy storage converter includes an H-bridge unit T1, a supporting capacitor C, a DC-DC converter T2, and an energy storage unit BAT.

[0059] The H-bridge unit T1 consists of two parallel bridge arms, each with two switching transistors connected in series. When the system is connected to the grid, the H-bridge unit T1 operates in rectification mode to provide energy to the supporting capacitor C; when the system is off-grid, the H-bridge unit T1 outputs the target voltage to drive the motor.

[0060] The supporting capacitor C includes, but is not limited to, film capacitors, and can be a single film capacitor or multiple film capacitors connected in series and parallel. The supporting capacitor C is used to filter out switching ripple during system operation and to provide a stable DC bus voltage.

[0061] The DC-DC converter T2 includes a filter inductor L and two switching transistors connected in series. One end of the filter inductor L is connected between the two switching transistors, and the other end is connected to the energy storage unit BAT. When the system is connected to the grid, the DC-DC converter T2 charges the energy storage unit BAT; when the system is off-grid, the energy from the energy storage unit BAT provides energy for the supporting capacitor C.

[0062] When the system is connected to the grid, the energy storage unit BAT stores energy; when the system is off-grid, the energy storage unit BAT feeds energy back to the motor output power or absorbs energy during the motor braking process.

[0063] exist Figure 2 In this process, the switching transistors include, but are not limited to, IGBTs, IGCTs, MOSFETs, etc., with IGBTs being the preferred choice.

[0064] Figure 3 This is a block diagram of the grid-connected charging process control of a cascaded multilevel energy storage system provided in an embodiment of this application.

[0065] like Figure 3 As shown, the control of the grid-connected charging process of the cascaded multilevel energy storage system includes the grid-connected rectification control link of the cascaded H-bridge topology and the DC-DC converter control link.

[0066] The cascaded H-bridge topology grid-connected rectifier control circuit is used to stabilize the support capacitor voltage of each three-phase power module and absorb energy from the grid to charge the energy storage unit of each power module.

[0067] The DC-DC converter control circuit is used to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and voltage regulation control after charging is completed.

[0068] Figure 4 This is a block diagram of grid-connected rectification control for a cascaded H-bridge topology provided in an embodiment of this application.

[0069] like Figure 4 As shown, the grid-connected rectifier control stage of the cascaded H-bridge topology adopts a dual closed-loop vector control strategy for voltage and current, combined with phase-to-phase equalization control of the supporting capacitor voltage and the nearest level approximation alternating wave generation strategy, to achieve phase-to-phase and in-phase equalization control of the supporting capacitor voltage of the power module.

[0070] In voltage-current dual closed-loop vector control, the outer loop is the voltage loop, supporting the capacitor voltage setpoint. The average voltage U of the three-phase support capacitor dc_abc After the difference is calculated, the active current setpoint of the converter is generated through the PI regulator. Converter reactive current setting value Typically set to 0, achieving unity power factor control. The inner loop is the current loop, including the d-axis current loop and the q-axis current loop. The output of the PI regulator in the d / q-axis current loop (converter active current setpoint) is... With converter active power feedback value i d After being combined, the output of the PI regulator becomes the converter reactive current setpoint. With the reactive power feedback value i of the converter q (The output after combining with the PI regulator) combined with the feedforward quantity V gd V gq (dq-axis component of grid voltage) and cross-decoupling quantity ωLsI q ,ωLsI d Generate the d / q axis component V of the target voltage for wave emission. d V q The original value V of the three-phase target voltage is generated by dq / abc transformation. a V b V c The original value of the three-phase wave generation target voltage is then corrected by the phase-to-phase equalization control unit of the supporting capacitor voltage to generate the final value of the three-phase wave generation target voltage. Among them, the active and reactive power feedback values ​​i of the converter d i q It is based on the grid current (i a i b i c The angle θ generated by the phase-locked loop is generated through the abc / dq transformation; the feedforward quantity V gd V gq It is based on the grid voltage (V) ga V gb V gc The angle θ generated by the phase-locked loop is generated through the abc / dq transformation.

[0071] like Figure 5 The diagram shows the block diagram for the phase-to-phase equalization control of the supporting capacitor voltage. The supporting capacitor voltage phase-to-phase equalization control unit first sets the supporting capacitor voltage value... The average value U of the supporting capacitor voltages A, B, and C in each phase dc_a U dc_b U dc_c The difference is adjusted by a PI controller; then the output of the PI controller is multiplied by the cosine values ​​of the three-phase voltages cosθ, cos(θ-2π / 3), and cos(θ+2π / 3) respectively to obtain the equalization adjustment amount; finally, the original value of the three-phase wave transmission target voltage V is... a V b V c Subtracting the equalization adjustment amount for the corresponding phase, we obtain the final value of the three-phase wave generation target voltage. The aforementioned phase-to-phase voltage equalization control unit for the supporting capacitor can effectively achieve phase-to-phase voltage equalization control.

[0072] The recent level waveform calculation module calculates the final value of the three-phase waveform target voltage. Generate drive signals for N sets of H-bridge units in each phase arm of the three-phase bridge arm, and sort the N power modules of each phase arm in descending order of supporting capacitor voltage value. Then, based on the instantaneous power P of each phase... i =V i * *i i The directions (positive or negative) of (i = a, b, c) are sequentially assigned to the drive signals PWM_abc of the H-bridge units according to the above sorting results. When the instantaneous power is positive (power flows into the bridge arm of the energy storage converter), the required number of power modules are selected first from the power modules with low supporting capacitor voltage values, so that the H-bridge unit T1 of the selected power modules outputs an effective level (positive or negative level), and the H-bridge unit T1 of the remaining power modules outputs a zero level. When the instantaneous power is negative (power flows out of the bridge arm of the energy storage converter), the required number of power modules are selected first from the power modules with high supporting capacitor voltage values, so that the H-bridge unit T1 of the selected power modules outputs an effective level (positive or negative level), and the H-bridge unit T1 of the remaining power modules outputs a zero level. This achieves grid-connected modulation operation of the converter, and through the above sorting process, achieves inter-stage equalization control of the supporting capacitor voltage of each power module at each stage of each phase.

[0073] Figure 6 This is a block diagram of a DC-DC converter control provided in an embodiment of this application.

[0074] like Figure 6 As shown, the control loop of the DC-DC converter consists of a phase charging current processing loop, a unit charging current processing loop, a constant current and voltage stabilization mode switching loop, a voltage control loop, a current control loop, and a waveform control loop.

[0075] During the grid-connected charging process of the energy storage converter, the operation of the DC-DC converter of each power module is divided into two stages: constant current charging operation of the energy storage unit and voltage stabilization operation of the energy storage unit.

[0076] When the supporting capacitor voltage is below a set threshold, the DC-DC converter operates in the constant current charging phase. During this phase, the voltage control loop is disabled, and only the current control loop operates, controlling the current command of each power module's DC-DC converter. The given charging current is generated after the three-phase charging current processing stages (A, B, and C) and the unit charging current processing stages of each power module.

[0077] When the supporting capacitor voltage exceeds a set threshold, the DC-DC converter operates in the energy storage unit voltage regulation phase. During this phase, the voltage control loop and current control loop work together to stabilize the energy storage unit voltage at the target voltage setpoint u. ref Specifically, the target voltage setting value u of the energy storage unit of each power module. ref With the voltage feedback value u of the energy storage unit fb The difference, after passing through the voltage loop PI regulator and the constant current and voltage stabilization mode switching stage, generates the energy storage unit charging current setpoint i. L_ref Then i L_ref Feedback value i of the charging current of the energy storage unit L The difference is processed by a current loop PI regulator, and the output of the current loop PI regulator is compared with the energy storage unit voltage feedback value u. fb Add them together to generate the DC-DC converter modulation signal u. pwm Finally, the driver generation module generates the PWM drive signal required by the DC-DC converter.

[0078] The three-phase charging current processing steps for phases A, B, and C can be referenced. Figure 7 As shown, firstly, the charging current scaling factor calculation unit calculates the charging current based on the total average voltage U of all three-phase energy storage units. stor_abc and the average voltage U of each phase energy storage unit stor_a U stor_b U stor_c The scaling factor k of the three-phase charging current is generated respectively. a k b k c Then the scaling factor is set to the same value as the charging current of the energy storage unit. Multiply to obtain the three-phase charging current setpoint. This process is used to achieve consistency in the total charging power of the three-phase energy storage units, and can quickly achieve equilibrium when there are differences in the average voltage of the three-phase energy storage units. It also facilitates the phase-to-phase equilibrium control of the voltage of the three-phase supporting capacitors.

[0079] The unit charging current processing steps for each power module can be referenced. Figure 8 As shown, with Figure 1 Taking phase A of the bridge arm as an example, the charging current scaling factor calculation unit calculates the charging current scaling factor based on the total average voltage U of all energy storage units in phase A. stor_a and the voltage sampling values ​​U of N energy storage units stor_a1 U stor_a2 、…、U stor_aN The scaling factor k of the charging current of each energy storage unit is generated respectively. a1 k a2 ..., k aN Then, the scaling factor of the charging current of each energy storage unit in phase A is compared with the set value of the charging current of the energy storage unit in phase A. Multiplying these values ​​yields the setpoints for the charging current of each energy storage unit in phase A. The same process is applied to phases B and C, and will not be repeated here. This process is used to achieve consistency in charging power between energy storage units in each phase, and can quickly achieve balance when there are differences in voltage between energy storage units in each phase. It also facilitates the inter-stage balanced control of the voltage of the supporting capacitors in each phase.

[0080] Figure 9 This is a schematic diagram of the grid connection control method for a cascaded multilevel energy storage system provided in the embodiments of this application.

[0081] like Figure 9 As shown, the method includes the following steps:

[0082] S11. Perform phase-to-phase equalization control of the supporting capacitor voltage on the original value of the three-phase wave generation target voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, and according to the result of sorting the N power modules of each phase bridge arm by the magnitude of the supporting capacitor voltage value, the drive signals of the H-bridge unit are allocated sequentially.

[0083] S12. Control the DC-DC converter in each power module to obtain energy from the supporting capacitor to charge the energy storage unit with constant current, and stabilize the voltage of the energy storage unit at the target voltage setting value of the energy storage unit.

[0084] In one example, the original value of the three-phase wave generation target voltage is generated by using voltage and current dual closed-loop vector control based on the grid current, grid voltage, and supporting capacitor voltage setpoints.

[0085] In one example, the phase-to-phase voltage equalization control of the supporting capacitor includes:

[0086] First, the difference between the set value of the supporting capacitor voltage and the average value of each phase of the supporting capacitor voltage is adjusted by a PI regulator; then, the output of the PI regulator is multiplied by the cosine value of the three-phase voltage of the corresponding phase to obtain the balancing adjustment amount; finally, the original value of the three-phase wave generation target voltage is subtracted from the balancing adjustment amount of the corresponding phase to obtain the final value of the three-phase wave generation target voltage.

[0087] In one example, the process of allocating drive signals to the H-bridge unit sequentially based on the final value of the three-phase target voltage and the direction of the instantaneous power of each phase, according to the result of sorting the N power modules of each phase arm by the magnitude of the supporting capacitor voltage, includes:

[0088] When the instantaneous power is positive, select the required number of power modules from the power modules with low supporting capacitor voltage values, so that the H-bridge unit of the selected power modules outputs an effective level, and the H-bridge unit of the remaining power modules outputs a zero level.

[0089] When the instantaneous power is negative, select the required number of power modules from the power modules with high supporting capacitor voltage values, so that the H-bridge unit of the selected power modules outputs an effective level, and the H-bridge unit of the remaining power modules outputs a zero level.

[0090] In one example, controlling the DC-DC converters in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the target voltage set value includes:

[0091] When the voltage of the supporting capacitor is lower than the set threshold, the DC-DC converter in each power module is controlled to obtain energy from the supporting capacitor to charge the energy storage unit with constant current.

[0092] When the voltage of the supporting capacitor is higher than the set threshold, the voltage of the energy storage unit will be stabilized at the target voltage set value of the energy storage unit.

[0093] In one example, when the supporting capacitor voltage is lower than a set threshold, the current command of the DC-DC converter in each power module comes from the charging current setting value generated after passing through the phase charging current processing stage and the unit charging current processing stage in sequence.

[0094] In one example, the phase charging current processing step includes:

[0095] Based on the total average voltage of all three-phase energy storage units and the average voltage of each phase energy storage unit, a scaling factor for the three-phase charging current is generated. Then, the scaling factor for the three-phase charging current is multiplied by the unified set value of the energy storage unit charging current to obtain the set value of the three-phase charging current.

[0096] In one example, the unit charging current processing step includes:

[0097] Based on the total average voltage of all energy storage units in a certain phase and the voltage sampling values ​​of N energy storage units, a scaling factor for the charging current of each energy storage unit is generated. Then, the scaling factor for the charging current of each energy storage unit in that phase is multiplied by the set value of the charging current of the energy storage unit in that phase to obtain the set value of the charging current of each energy storage unit in that phase.

[0098] In one example, when the supporting capacitor voltage is higher than a set threshold, the difference between the target voltage set value and the energy storage unit voltage feedback value of each power module is processed by a voltage loop PI regulator and a constant current and voltage stabilization mode switching circuit to generate an energy storage unit charging current set value. Then, the difference between the energy storage unit charging current set value and the energy storage unit charging current feedback value is processed by a current loop PI regulator, and the output of the current loop PI regulator is added to the energy storage unit voltage feedback value to generate a DC-DC converter modulation signal. Finally, the drive generation module generates the drive signal required by the DC-DC converter.

[0099] Furthermore, embodiments of this application also provide a cascaded multilevel energy storage system, the cascaded multilevel energy storage system including an energy storage converter and a controller;

[0100] The energy storage converter includes a three-phase bridge arm, each phase bridge arm includes N cascaded power modules, each power module includes an H-bridge unit, a supporting capacitor, a DC-DC converter and an energy storage unit;

[0101] The controller is configured to perform phase-to-phase equalization control of the supporting capacitor voltage on the original value of the three-phase wave generation target voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, the controller sequentially distributes the drive signals of the H-bridge unit according to the result of sorting the N power modules of each phase arm by the magnitude of the supporting capacitor voltage value. The controller controls the DC-DC converter in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the set value of the energy storage unit target voltage.

[0102] Furthermore, embodiments of this application also provide a computer-readable storage medium storing at least one line of program code, which is loaded and executed by a processor to implement the grid-connected control method for the cascaded multilevel energy storage system described above.

[0103] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. A grid-connected control method for a cascaded multilevel energy storage system, wherein the cascaded multilevel energy storage system includes an energy storage converter, the energy storage converter includes three-phase bridge arms, each phase bridge arm includes N cascaded power modules, each power module includes an H-bridge unit, a supporting capacitor, a DC-DC converter, and an energy storage unit; characterized in that, The control method includes: The original value of the three-phase wave generation target voltage is subjected to phase-to-phase equalization control of the supporting capacitor voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, the drive signals of the H-bridge unit are sequentially allocated according to the result of sorting the N power modules of each phase bridge arm by the magnitude of the supporting capacitor voltage value. The DC-DC converters in each power module are controlled to obtain energy from the supporting capacitor to charge the energy storage unit at a constant current, and the voltage of the energy storage unit is stabilized at the target voltage set value of the energy storage unit. The control of the DC-DC converters in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the target voltage set value includes: When the voltage of the supporting capacitor is lower than the set threshold, the DC-DC converter in each power module is controlled to obtain energy from the supporting capacitor to charge the energy storage unit with constant current. When the voltage of the supporting capacitor is higher than the set threshold, the voltage of the energy storage unit will be stabilized at the target voltage set value of the energy storage unit.

2. The method according to claim 1, characterized in that, Based on the grid current, grid voltage, and supporting capacitor voltage settings, voltage and current dual closed-loop vector control is used to generate the original value of the three-phase wave generation target voltage.

3. The method according to claim 1, characterized in that, The phase-to-phase voltage equalization control of the supporting capacitor includes: First, the difference between the set value of the supporting capacitor voltage and the average value of each phase of the supporting capacitor voltage is adjusted by a PI regulator; then, the output of the PI regulator is multiplied by the cosine value of the three-phase voltage of the corresponding phase to obtain the balancing adjustment amount; finally, the original value of the three-phase wave generation target voltage is subtracted from the balancing adjustment amount of the corresponding phase to obtain the final value of the three-phase wave generation target voltage.

4. The method according to claim 1, characterized in that, The method involves allocating drive signals to the H-bridge unit sequentially based on the final value of the three-phase target voltage and the direction of the instantaneous power of each phase, according to the result of sorting the N power modules of each phase arm by the magnitude of the supporting capacitor voltage. This includes: When the instantaneous power is positive, select the required number of power modules from the power modules with low supporting capacitor voltage values, so that the H-bridge unit of the selected power modules outputs an effective level, and the H-bridge unit of the remaining power modules outputs a zero level. When the instantaneous power is negative, select the required number of power modules from the power modules with high supporting capacitor voltage values, so that the H-bridge unit of the selected power modules outputs an effective level, and the H-bridge unit of the remaining power modules outputs a zero level.

5. The method according to claim 1, characterized in that, When the supporting capacitor voltage is lower than the set threshold, the current command of the DC-DC converter in each power module comes from the charging current setting value generated after passing through the phase charging current processing stage and the unit charging current processing stage in sequence.

6. The method according to claim 5, characterized in that, The phase charging current processing step includes: Based on the total average voltage of all three-phase energy storage units and the average voltage of each phase energy storage unit, a scaling factor for the three-phase charging current is generated. Then, the scaling factor for the three-phase charging current is multiplied by the unified set value of the energy storage unit charging current to obtain the set value of the three-phase charging current.

7. The method according to claim 6, characterized in that, The unit charging current processing step includes: Based on the total average voltage of all energy storage units in a certain phase and the voltage sampling values ​​of N energy storage units, a scaling factor for the charging current of each energy storage unit is generated. Then, the scaling factor for the charging current of each energy storage unit in that phase is multiplied by the set value of the charging current of the energy storage unit in that phase to obtain the set value of the charging current of each energy storage unit in that phase.

8. The method according to claim 1, characterized in that, When the supporting capacitor voltage is higher than the set threshold, the difference between the target voltage set value of the energy storage unit of each power module and the voltage feedback value of the energy storage unit is used to generate the charging current set value of the energy storage unit after passing through the voltage loop PI regulator and the constant current and voltage stabilization mode switching circuit. Then, the difference between the set value of the energy storage unit charging current and the feedback value of the energy storage unit charging current is processed by the current loop PI regulator, and the output of the current loop PI regulator is added to the energy storage unit voltage feedback value to generate a DC-DC converter modulation signal. Finally, the drive generation module generates the drive signals required by the DC-DC converter.

9. A cascaded multilevel energy storage system, characterized in that, The cascaded multilevel energy storage system includes an energy storage converter and a controller; The energy storage converter includes a three-phase bridge arm, each phase bridge arm includes N cascaded power modules, each power module includes an H-bridge unit, a supporting capacitor, a DC-DC converter and an energy storage unit; The controller is configured to perform phase-to-phase equalization control of the supporting capacitor voltage on the original value of the three-phase wave generation target voltage to generate the final value of the three-phase wave generation target voltage. Based on the final value of the three-phase wave generation target voltage and the direction of the instantaneous power of each phase, the controller distributes the drive signals of the H-bridge unit in sequence according to the result of sorting the N power modules of each phase arm by the magnitude of the supporting capacitor voltage value. The controller controls the DC-DC converter in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the target voltage set value of the energy storage unit. The control of the DC-DC converters in each power module to obtain energy from the supporting capacitor to perform constant current charging of the energy storage unit and stabilize the voltage of the energy storage unit at the target voltage set value includes: When the voltage of the supporting capacitor is lower than the set threshold, the DC-DC converter in each power module is controlled to obtain energy from the supporting capacitor to charge the energy storage unit with constant current. When the voltage of the supporting capacitor is higher than the set threshold, the voltage of the energy storage unit will be stabilized at the target voltage set value of the energy storage unit.

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