A balanced charging and discharging control method for cascaded multi-level energy storage converter

Abstract

The present invention provides a balanced charge and discharge control method for a cascaded multi-level energy storage converter. The energy storage elements of the cascaded multi-level energy storage converter are distributed in each submodule. Since the initial charge state of each energy storage element is random, it is difficult to ensure the balance of the charge state of each energy storage element during the charging and discharging stages, resulting in low energy utilization and even the risk of overcharging or over-discharging the energy storage element. The present invention adjusts the charge and discharge power of the submodule by controlling the AC side power of each submodule. In particular, the submodule with a large charge state deviation can be adjusted independently, and the charge state can be quickly balanced, thereby improving the energy utilization and reliability of the energy storage converter.

Description

Technical Field

[0001] The present invention belongs to the technical field of multi-level energy storage converter control, and in particular relates to a cascaded multi-level energy storage converter balanced charging and discharging control method. Background Art

[0002] Cascaded multi-level energy storage converters have been widely used in applications that require high-voltage and large-capacity energy storage systems, such as renewable energy power generation and high-speed electromagnetic drive.

[0003] At present, cascaded multi-level energy storage converters are mainly modular multi-level converters and cascaded H-bridge multi-level converters. In traditional energy storage systems, each power sub-module generally uses energy storage elements directly connected in parallel to the H-bridge DC side support capacitor, such as Chinese invention patent CN108923518A. However, its voltage operating range is narrow, which limits the discharge depth of the energy storage element and affects its energy utilization rate. At the same time, the larger current ripple will also affect the service life of the energy storage element. Chinese invention patent CN105897018A adopts a two-stage conversion circuit topology. Each power sub-module adds a DC-DC converter to connect the energy storage element and the support capacitor, which increases the voltage operating range of the energy storage element, and can control the stability of the support capacitor voltage, better ensuring the quality of the AC side output voltage.

[0004] When the number of power submodules in the energy storage system is large, it is more reasonable for the cascaded multi-level energy storage converter to adopt the nearest level approximation modulation, such as the Chinese invention patent CN110556852A, because the more modules there are, the more ideal the waveform obtained by the nearest level approximation modulation is, and it is easy to understand, calculate and implement. However, it will cause a large difference in the time when each power submodule outputs a positive level or a negative level, that is, the power on the AC side of each module is inconsistent, resulting in a large difference in the charging and discharging power of the energy storage element.

[0005] Each power submodule of the cascaded multilevel energy storage converter is independent of each other, and the energy storage elements are distributed in each submodule. Since the initial charge state of each energy storage element is random, it is difficult to ensure the balance of the charge state of each energy storage element during the charging and discharging stages. This makes the energy of the energy storage elements between modules inconsistent, resulting in low energy utilization, seriously affecting their service life, and even causing the energy storage elements to be overcharged or over-discharged. Summary of the invention

[0006] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a balanced charge and discharge control method for a cascaded multi-level energy storage converter. Aiming at the problem that the charge and discharge power of the energy storage elements of each submodule is inconsistent during the nearest level approach modulation, and the state of charge SOC distribution of each energy storage element in the phase is uneven, the present invention realizes the rotation operation of each submodule according to the sorting result of the state of charge of the energy storage element of each submodule, and then controls the AC side power of each submodule, and adjusts the charge and discharge power of the submodule. In particular, the submodule with a large deviation in the state of charge SOC can be adjusted independently to quickly achieve a balanced state of charge.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A balanced charging and discharging control method for a cascaded multi-level energy storage converter, wherein the energy storage converter is a two-stage conversion type cascaded multi-level energy storage converter, with a total of A, B, and C three-phase bridge arms, each bridge arm is composed of N+m power sub-modules, m is the number of redundant modules, and N and m are positive integers; each power sub-module is composed of an energy storage element, a DC-DC circuit, a support capacitor, and an H-bridge circuit; the energy storage element is connected to the support capacitor through the DC-DC circuit, and the support capacitor is connected in parallel between the DC-DC circuit and the H-bridge circuit; when charging on the grid, the AC side of the H-bridge circuit is connected to the grid; when running off the grid , the AC side of the H-bridge circuit is connected to the AC load; the control system includes an AC control unit, a modulation unit and a submodule control unit; the AC control unit controls the charging and discharging power of the AC side of the energy storage converter; the modulation unit receives the modulation voltage sent by the AC control unit, uses the nearest level approximation modulation to obtain the number of submodules that need to be put into operation at each moment, combines the charging and discharging state and selects the corresponding submodule to output positive level, negative level and zero level according to the charge state sorting result of the energy storage element; the submodule control unit independently controls each power submodule;

[0009] The control method comprises the following steps:

[0010] Step 1: in the modulation unit, the modulation voltage of each phase is modulated by the nearest level approximation, and the ratio of the reference voltage of each phase to the voltage average value of the support capacitors of all power sub-modules of the phase is calculated in real time, rounded to the nearest integer, and its absolute value n is obtained;

[0011] Step 2: Sort the state of charge SOC of the energy storage elements of each power submodule;

[0012] Step 3: According to the absolute value n of step 1 and the sorting result of the state of charge SOC of the energy storage element in step 2, the corresponding power submodule is selected to output a positive level, a negative level and a zero level.

[0013] Furthermore, in the grid-connected charging stage: the power sub-module SMy with the largest state of charge SOC is made to output a level opposite to the phase voltage, and at the same time, the power sub-module SMz with the smallest state of charge SOC is made to output a level completely opposite to that of the power sub-module SMy, that is, the combined level of the power sub-module SMy and the power sub-module SMz is zero.

[0014] Further, when the phase voltage ug>0: if n≤N-2, the power sub-module SMy outputs a negative level, the power sub-module SMz outputs a positive level, and n power sub-modules with lower state of charge SOC are selected in sequence from the remaining power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with lower state of charge SOC are directly selected in sequence from all the power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels.

[0015] Further, when the phase voltage ug<0: if n≤N-2, the power sub-module SMy outputs a positive level, the power sub-module SMz outputs a negative level, and n power sub-modules with lower state of charge SOC are selected in sequence from the remaining power sub-modules to output negative levels, and the remaining power sub-modules including the redundant modules output zero levels; if n>N-2, n power sub-modules with lower state of charge SOC are directly selected in sequence from all the power sub-modules to output negative levels, and the remaining power sub-modules including the redundant modules output zero levels; this makes the power sub-module SMy in a discharging state and the power sub-module SMz in a charging state during the charging stage.

[0016] Furthermore, in the off-grid discharge operation stage: the power sub-module SMz with the smallest state of charge SOC is made to output a level opposite to the phase voltage, and at the same time, the power sub-module SMy with the largest state of charge SOC is made to output a level completely opposite to that of the power sub-module SMz, that is, the combined level of the power sub-module SMz and the power sub-module SMy is zero.

[0017] Further, when the phase voltage ug>0: if n≤N-2, the power sub-module SMz outputs a negative level, the power sub-module SMy outputs a positive level, and n power sub-modules with higher state of charge SOC are selected in sequence from the remaining power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with higher state of charge SOC are directly selected in sequence from all the power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels.

[0018] Furthermore, when the phase voltage ug<0: if n≤N-2, the power submodule SMz outputs a positive level, the power submodule SMy outputs a negative level, and n power submodules with higher state of charge SOC are selected in order from the remaining power submodules to output negative levels, and the remaining power submodules including the redundant modules output zero levels; if n>N-2, n power submodules with higher state of charge SOC are directly selected in order from all power submodules to output negative levels, and the remaining power submodules including the redundant modules output zero levels. This makes the power submodule SMz in the charging state and the power submodule SMy in the discharging state during the discharge stage.

[0019] Beneficial effects:

[0020] The cascaded multi-level energy storage converter balanced charge and discharge control method of the present invention can flexibly adjust the submodule charge and discharge power by controlling the AC side power of each power submodule. In particular, the submodules with large charge state deviations can be adjusted independently to achieve rapid balance of the charge state of the energy storage element, thereby improving the energy utilization and reliability of the energy storage converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The circuit topology diagram of the cascaded multi-level energy storage converter of the present invention; wherein (a) is the circuit topology diagram during grid-connected charging; and (b) is the circuit topology diagram during off-grid operation.

[0022] Figure 2 A circuit topology diagram of a power submodule in a cascaded multi-level energy storage converter of the present invention;

[0023] Figure 3 A schematic diagram of a nearest level approximation modulation strategy for a cascaded multi-level energy storage converter according to the present invention;

[0024] Figure 4 The invention discloses a balanced charge and discharge control method for a cascaded multi-level energy storage converter; wherein (a) is a balanced charge and discharge control method for grid-connected charging; and (b) is a balanced charge and discharge control method for off-grid operation. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] like Figure 1-4As shown, the present invention provides a balanced charge and discharge control method for a cascaded multi-level energy storage converter. The energy storage converter applied to the present invention is a two-stage conversion type cascaded multi-level energy storage converter, with a total of three-phase bridge arms A, B, and C, each bridge arm is composed of N+m power sub-modules, m is the number of redundant modules, N and m are positive integers; each power sub-module is composed of an energy storage element, a DC-DC circuit, a support capacitor, and an H-bridge circuit; the energy storage element is connected to the support capacitor through the DC-DC circuit, and the support capacitor is connected in parallel between the DC-DC circuit and the H-bridge circuit. When charging in grid connection, the AC side of the H-bridge circuit is connected to the power grid; when operating off-grid, the AC side of the H-bridge circuit is connected to the AC load. The control system mainly includes three parts: an AC control unit, a modulation unit, and a submodule control unit. The AC control unit controls the charging and discharging power of the AC side of the energy storage converter. The modulation unit receives the modulation voltage sent by the AC control unit, uses the nearest level approximation modulation to obtain the number of submodules that need to be put into operation at each moment, combines the charge and discharge state and selects the corresponding submodule to output positive level, negative level and zero level according to the charge state sorting result of the energy storage element. The submodule control unit independently controls each submodule, on the one hand, drives each submodule IGBT device according to the switching state of each submodule received from the modulation unit, and on the other hand, performs closed-loop control on the support capacitor voltage to maintain the stability of the support capacitor voltage.

[0027] The cascaded multi-level energy storage converter balanced charge and discharge control method of the present invention specifically comprises the following steps:

[0028] Step 1: In the modulation unit, the modulation voltage of each phase is modulated by the nearest level approximation, that is, the ratio of the reference voltage of each phase to the average value of the support capacitor voltage of all modules of this phase is calculated in real time, rounded off, and its absolute value n is obtained.

[0029] Step 2: Sort the state of charge (SOC) of the energy storage elements of each power submodule.

[0030] Step 3: According to the calculation result of step 1 and the sorting result of the state of charge SOC of the energy storage element in step 2, the corresponding submodule is selected to output positive level, negative level and zero level.

[0031] In the grid-connected charging stage: the power submodule SMy with the largest state of charge SOC can be made to output a level opposite to the phase voltage, and at the same time, the power submodule SMz with the smallest state of charge SOC can be made to output a level completely opposite to that of the power submodule SMy, that is, the combined level of the power submodule SMy and the power submodule SMz is zero. That is, when the phase voltage ug>0: if n≤N-2, the power submodule SMy outputs a negative level, the power submodule SMz outputs a positive level, and n power submodules with lower state of charge SOC are selected in sequence from the remaining power submodules to output positive levels, and the remaining power submodules output zero levels (including redundant modules); if n>N-2, n power submodules with lower state of charge SOC are directly selected in sequence from all power submodules to output positive levels, and the remaining power submodules output zero levels. Correspondingly, when the phase voltage ug<0: if n≤N-2, the power submodule SMy outputs a positive level, the power submodule SMz outputs a negative level, and n power submodules with lower state of charge SOC are selected in order from the remaining power submodules to output negative levels, and the remaining power submodules output zero levels (including redundant modules); if n>N-2, n power submodules with lower state of charge SOC are directly selected in order from all power submodules to output negative levels, and the remaining power submodules output zero levels. This makes the power submodule SMy in the discharging state and the power submodule SMz in the charging state during the charging stage.

[0032] In the off-grid discharge operation stage: the power submodule SMz with the smallest state of charge SOC can be made to output a level opposite to the phase voltage, and at the same time, the power submodule SMy with the largest state of charge SOC can be made to output a level completely opposite to the power submodule SMz, that is, the combined level of the power submodule SMz and the power submodule SMy is zero. That is, when the phase voltage ug>0: if n≤N-2, the power submodule SMz outputs a negative level, the power submodule SMy outputs a positive level, and n power submodules with higher state of charge SOC are selected in sequence from the remaining power submodules to output positive levels, and the remaining power submodules output zero levels (including redundant modules); if n>N-2, n power submodules with higher state of charge SOC are directly selected in sequence from all power submodules to output positive levels, and the remaining power submodules output zero levels. Correspondingly, when the phase voltage ug<0: if n≤N-2, the power submodule SMz outputs a positive level, the power submodule SMy outputs a negative level, and n power submodules with higher state of charge SOC are selected in order from the remaining power submodules to output negative levels, and the remaining power submodules output zero levels (including redundant modules); if n>N-2, n power submodules with higher state of charge SOC are directly selected in order from all power submodules to output negative levels, and the remaining power submodules output zero levels. This makes the power submodule SMz in the charging state and the power submodule SMy in the discharging state during the discharge stage.

[0033] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A balanced charge and discharge control method for a cascaded multi-level energy storage converter, characterized in that: The energy storage converter is a two-stage conversion type cascade multi-level energy storage converter, with a total of three-phase bridge arms A, B, and C, each bridge arm is composed of N+m power sub-modules, m is the number of redundant modules, and N and m are positive integers; each power sub-module is composed of an energy storage element, a DC-DC circuit, a support capacitor, and an H-bridge circuit; the energy storage element is connected to the support capacitor through the DC-DC circuit, and the support capacitor is connected in parallel between the DC-DC circuit and the H-bridge circuit; when connected to the grid for charging, the AC side of the H-bridge circuit is connected to the grid; when off-grid, the AC side of the H-bridge circuit is connected to the AC load; the control system includes an AC control unit, a modulation unit, and a sub-module control unit; the AC control unit controls the charging and discharging power of the AC side of the energy storage converter; the modulation unit receives the modulation voltage sent by the AC control unit, adopts the nearest level approximation modulation to obtain the number of sub-modules that need to be put into operation at each time, combines the charging and discharging state and selects the corresponding sub-module to output positive level, negative level and zero level according to the sorting result of the energy storage element charge state; the sub-module control unit independently controls each power sub-module; The control method comprises the following steps: Step 1: in the modulation unit, the modulation voltage of each phase is modulated by the nearest level approximation, and the ratio of the reference voltage of each phase to the voltage average value of the support capacitors of all power sub-modules of the phase is calculated in real time, rounded to the nearest integer, and its absolute value n is obtained; Step 2: Sort the state of charge SOC of the energy storage elements of each power submodule; Step 3: According to the absolute value n of step 1 and the sorting result of the state of charge SOC of the energy storage element in step 2, the corresponding power submodule is selected to output a positive level, a negative level and a zero level.

2. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 1, characterized in that: During the grid-connected charging stage: the power sub-module SMy with the largest state of charge SOC is made to output a level opposite to the phase voltage. At the same time, the power sub-module SMz with the smallest state of charge SOC is made to output a level completely opposite to that of the power sub-module SMy, that is, the combined level of the power sub-module SMy and the power sub-module SMz is zero.

3. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 2, characterized in that: When the phase voltage ug>0: if n≤N-2, the power sub-module SMy outputs a negative level, the power sub-module SMz outputs a positive level, and n power sub-modules with lower state of charge SOC are selected in sequence from the remaining power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with lower state of charge SOC are directly selected in sequence from all power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels.

4. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 2, characterized in that: When the phase voltage ug<0: if n≤N-2, the power sub-module SMy outputs a positive level, the power sub-module SMz outputs a negative level, and n power sub-modules with lower state of charge SOC are selected in sequence from the remaining power sub-modules to output negative levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with lower state of charge SOC are directly selected in sequence from all power sub-modules to output negative levels, and the remaining power sub-modules including redundant modules output zero levels; this makes the power sub-module SMy in a discharging state and the power sub-module SMz in a charging state during the charging stage.

5. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 2, characterized in that: During the off-grid discharge operation stage: the power sub-module SMz with the smallest state of charge SOC is made to output a level opposite to the phase voltage. At the same time, the power sub-module SMy with the largest state of charge SOC is made to output a level completely opposite to that of the power sub-module SMz, that is, the combined level of the power sub-module SMz and the power sub-module SMy is zero.

6. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 2, characterized in that: When the phase voltage ug>0: if n≤N-2, the power sub-module SMz outputs a negative level, the power sub-module SMy outputs a positive level, and n power sub-modules with higher state of charge SOC are selected in sequence from the remaining power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with higher state of charge SOC are directly selected in sequence from all power sub-modules to output positive levels, and the remaining power sub-modules including redundant modules output zero levels.

7. The balanced charge and discharge control method for a cascaded multi-level energy storage converter according to claim 2, characterized in that: When the phase voltage ug<0: if n≤N-2, the power sub-module SMz outputs a positive level, the power sub-module SMy outputs a negative level, and n power sub-modules with higher state of charge SOC are selected in sequence from the remaining power sub-modules to output negative levels, and the remaining power sub-modules including redundant modules output zero levels; if n>N-2, n power sub-modules with higher state of charge SOC are directly selected in sequence from all power sub-modules to output negative levels, and the remaining power sub-modules including redundant modules output zero levels. Therefore, in the discharge stage, the power sub-module SMz is in a charging state and the power sub-module SMy is in a discharging state.

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