Power conversion system and method of controlling the same

Abstract

The application discloses a power conversion system and a control method thereof. The power conversion system comprises: n choppers, each of which comprises: a switching bridge arm; an inductor, a first end of which is connected to a midpoint of the switching bridge arm; and a first capacitor, which is connected in parallel with the switching bridge arm, wherein the switching bridge arms of each of the choppers are connected in parallel; and n direct current elements, which are in one-to-one correspondence with the n choppers, the direct current elements being direct current power sources or direct current loads, first ends of the n direct current elements being connected and second ends of the n direct current elements being connected to second ends of the inductors of the corresponding choppers, n being a natural number greater than or equal to 2. The application can efficiently and at low cost realize voltage and current regulation of direct current elements such as batteries, and improves the service life and safety of the batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power conversion, in particular to a power conversion system and a control method thereof. BACKGROUND

[0002] In a large power energy storage, a battery or a super capacitor usually needs thousands of low-voltage small units to be connected in series and in parallel to obtain a high-voltage large current required by the system. However, the series voltage imbalance and the parallel battery circulating current have restricted the reliability and usability of the system, and have seriously affected the investment benefit or the system safety. In the past two years, the fire accidents of energy storage power stations in many places reflect the urgent need of the electric energy storage system to improve the protection and safety performance of the system. In order to be safe, many energy storage systems have to limit the SOC (state of charge) range of battery charging and discharging, such as between 20% and 80%, to reserve 10%-20% margin for the voltage difference of the battery pack and the circulating current, resulting in an increase in investment cost.

[0003] In order to solve the above problems, there are mainly two methods at present: 1. A bidirectional DC-DC converter (chopper) is adopted, each battery pack is taken as an input, and a power supply or a load DC bus is taken as an output, so as to realize the bidirectional voltage and current conversion of the electric energy on both sides. The bidirectional DC-DC converter has performance advantages, including voltage and current regulation capability, input and output voltage conversion capability, and faster current turn-off capability. The disadvantage is the increase of the cost of elements and power loss for realizing high-voltage and large-current direct current conversion. 2. A series voltage compensation is adopted to regulate the direct current voltage and current, and a two-port DC-DC converter for voltage compensation is introduced into the direct current power supply system. The two-port DC-DC converter for voltage compensation has an input port and an output port, the output port is connected in series to the power supply circuit of the power supply system to provide voltage superposition compensation, and the input port provides compensation electric energy. When the amplitude of voltage regulation is small compared with the proportion of the power supply voltage, a small power DC-DC converter for voltage compensation can be used to regulate the total output voltage and current. Compared with the chopper with full power input and full power output, the power capacity of the compensation converter is greatly reduced, the volume and cost are reduced, and the power conversion loss is also reduced. This method is also called partial power conversion. The above-mentioned compensation DC-DC converter adopts an isolated converter, which includes a high-frequency transformer and a conversion process from direct current to high-frequency alternating current and then to direct current, so the number of elements is large, the volume is large, and the cost is high.

[0004] Therefore, it is necessary to find a power conversion system and a control method thereof to solve one or more of the above technical problems. SUMMARY

[0005] Therefore, the present application aims to provide a power conversion system, which adopts a small power non-isolated chopper with a special structure, realizes voltage and / or current regulation of a DC power source or a load, and further reduces the number and volume of components, cost and power loss.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a power conversion system is provided, which comprises n choppers, each of which comprises a switching bridge arm, an inductor with a first end connected to a midpoint of the switching bridge arm, and a first capacitor connected in parallel with the switching bridge arm, wherein the switching bridge arms of each chopper are connected in parallel, and

[0007] n DC elements corresponding to the n choppers, the DC elements being a DC power source or a DC load, the first ends of the n DC elements being connected and the second ends of the n DC elements being connected to the second ends of the inductors of the corresponding choppers, n being a natural number greater than or equal to 2.

[0008] According to another aspect of the present application, at least one of the n DC elements is a DC power source and at least one of the n DC elements is a DC load.

[0009] According to another aspect of the present application, the switching bridge arm comprises a first switch and a second switch connected in series, and a common connection point of the first switch and the second switch is the midpoint of the switching bridge arm.

[0010] According to another aspect of the present application, the switching bridge arm comprises a third switch, a fourth switch, a fifth switch and a sixth switch connected in series, and each of the choppers further comprises a flying capacitor electrically coupled between a common connection point of the third switch and the fourth switch and a common connection point of the fifth switch and the sixth switch, and a common connection point of the fourth switch and the fifth switch is the midpoint of the switching bridge arm.

[0011] According to another aspect of the present application, the DC power source comprises a battery, a rectified power source or a super capacitor.

[0012] According to another aspect of the present application, the DC power source further comprises a DC-DC converter electrically coupled between the inductor and the battery, the rectified power source or the super capacitor.

[0013] According to another aspect of the present application, the DC load comprises a battery, a super capacitor, a resistor, a DC / DC converter or a DC / AC converter.

[0014] According to another aspect of the present application, each of the choppers further comprises a second capacitor electrically coupled between the first end and / or the second end of the switching bridge arm and the second end of the inductor.

[0015] According to another aspect of the present application, the voltage of the first capacitor is lower than the voltage of the DC element.

[0016] According to another aspect of the present application, the power conversion system further comprises a compensation power supply connected in parallel with the switching bridge.

[0017] According to another aspect of the present application, one of the n DC elements is a DC terminal of an inverter.

[0018] According to another aspect of the present application, the power conversion system further comprises a control unit for controlling the switching bridge.

[0019] According to another aspect of the present application, the voltage of the DC terminal of the inverter is equal to a weighted average of the voltages of the remaining (n-1) DC elements.

[0020] According to another aspect of the present application, the weight of each of the remaining (n-1) DC elements in the weighted average is proportional to the ratio of the current flowing through it to the total current flowing through the remaining (n-1) DC elements.

[0021] According to another aspect of the present application, the remaining (n-1) DC elements are battery packs, and the control unit controls the current flowing through the remaining (n-1) DC elements.

[0022] According to another aspect of the present application, the voltage of the first capacitor is less than 50% of the rated voltage of the corresponding battery pack.

[0023] According to another aspect of the present application, the remaining (n-1) DC elements are photovoltaic cell strings.

[0024] According to another aspect of the present application, when the voltage of the DC terminal of the inverter is near the average voltage of the maximum power point (MPP) voltages of the (n-1) photovoltaic cell strings, the control unit controls each of the second end voltages of the inductors with the MPP voltage of each of the photovoltaic cell strings as a target value.

[0025] According to another aspect of the present application, a control method for a power conversion system is provided, the control method comprising the steps of:

[0026] providing n choppers, each of which comprises a switching bridge, an inductor having a first end connected to the midpoint of the switching bridge, and a first capacitor connected in parallel with the switching bridge, wherein the switching bridges of each of the choppers are connected in parallel;

[0027] The n direct current elements are connected to the second ends of the inductors of the n choppers respectively, and the first ends of the n direct current elements are connected to each other.

[0028] The current flowing through the direct current elements or the voltage of the direct current elements is adjusted by controlling the switch bridge arms.

[0029] According to another aspect of the present application, one of the n direct current elements is a direct current end of an inverter.

[0030] According to another aspect of the present application, the voltage of the direct current end of the inverter is controlled to be equal to a weighted average of the voltages of the remaining (n-1) direct current elements.

[0031] According to another aspect of the present application, the weight of the voltage of each of the remaining (n-1) direct current elements in the weighted average is the ratio of the current flowing through it to the total current flowing through the remaining (n-1) direct current elements.

[0032] According to another aspect of the present application, the remaining (n-1) direct current elements are battery packs.

[0033] According to another aspect of the present application, the voltage of the first capacitor is controlled to a fixed value.

[0034] According to another aspect of the present application, the fixed value is lower than the voltages of the battery packs and the direct current end of the inverter.

[0035] According to another aspect of the present application, the fixed value is less than 50% of the rated voltage of the battery packs.

[0036] According to another aspect of the present application, the remaining (n-1) direct current elements are photovoltaic cell strings.

[0037] According to another aspect of the present application, the voltage of the direct current end of the inverter is controlled to be near the average voltage of the MPP voltages of the (n-1) photovoltaic cell strings, and the voltage of the second end of each inductor is controlled with the MPP voltage of each photovoltaic cell string as a target value.

[0038] The power conversion system of the present application uses n choppers as small-power non-isolated converters to replace large-power converters to implement current regulation and circulating current suppression of direct current elements such as battery packs or super capacitors. The present application can be used in a multi-battery pack parallel energy storage system to realize low-cost circulating current suppression / current regulation / SOC optimization maintenance, improve battery service life and safety, and improve battery service life by about 15%.

[0039] The above description will be described in detail in the embodiments, and the technical solutions of the present application will be further explained. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to make the above and other objects, features, advantages and embodiments of the present application more apparent, the following will be described in detail with the accompanying drawings.

[0041] Figure 1 Circuit diagram of the power conversion system according to the first embodiment of the present application;

[0042] Figure 2 Circuit diagram of the power conversion system according to the second embodiment of the present application (with multiple types of DC power sources or DC loads);

[0043] Figure 3 Circuit diagram of the power conversion system according to the third embodiment of the present application (with filter capacitors arranged at different positions);

[0044] Figure 4 Circuit diagram of the power conversion system according to the fourth embodiment of the present application (with the inductance of the chopper connected to the negative pole of the DC power source or load, and the positive poles of the power source or load connected together);

[0045] Figure 5 Circuit diagram of the power conversion system according to the fifth embodiment of the present application (with a partial DC bus connected to a compensation power source);

[0046] Figure 6 Circuit diagram of the power conversion system according to the sixth embodiment of the present application;

[0047] Figure 7 Circuit diagram of the power conversion system according to the seventh embodiment of the present application;

[0048] Figure 8 Circuit diagram of the power conversion system according to the eighth embodiment of the present application;

[0049] Figure 9 Flow chart of the control method of the power conversion system according to an embodiment of the present application;

[0050] Figure 10 Circuit diagram of the power conversion system according to the ninth embodiment of the present application. DETAILED DESCRIPTION

[0051] For the purpose of making the description of the present application more comprehensive and complete, reference can be made to the accompanying drawings and the following various embodiments, in which the same or similar components are denoted by the same reference numerals. On the other hand, well-known components and steps are not described in the embodiments to avoid unnecessary limitations on the present application. In addition, for the purpose of simplifying the drawings, some known conventional structures and elements will be shown in the drawings in a simple schematic manner.

[0052] According to an embodiment of the present application, referring to Figure 1 , there is provided a power conversion system, which comprises n choppers CH1-CHn and n DC elements B1-Bn, n being a natural number greater than or equal to 2. Each of the choppers comprises a switching bridge arm, an inductor L1 having a first end connected to a midpoint of the switching bridge arm, and a first capacitor CB1 connected in parallel to the switching bridge arm, wherein the switching bridge arms of the choppers are connected in parallel. Further, each of the choppers comprises a switching bridge arm comprising a first switch Q1 and a second switch Q2 connected in series, an inductor L1 having a first end connected to a midpoint of the switching bridge arm, i.e. a common connection point of the first switch Q1 and the second switch Q2, and a first capacitor CB1 connected in parallel to the switching bridge arm, wherein the switching bridge arms of the choppers are connected in parallel. The n DC elements B1-Bn correspond to the n choppers CH1-CHn one-to-one, the DC elements being DC power sources or DC loads, the first ends of the n DC elements B1-Bn being connected and the second ends being connected to the second ends of the inductors L1 of the corresponding choppers, at least one of the n DC elements B1-Bn being a DC power source and at least one being a DC load. In some embodiments, the choppers work unidirectionally. In some embodiments, the choppers work bidirectionally. In some embodiments, some of the choppers work unidirectionally and some of the choppers work bidirectionally. Further, the switching bridge arms are connected in parallel between two ends LB+, LB- of a local DC bus BUS. The DC elements B1-Bn and the first capacitors CB1, i.e. the local DC bus BUS, are not directly connected.

[0053] In particular, the DC power source comprises a battery, a rectified power source or a super capacitor. The DC power source can further comprise a DC-DC converter electrically coupled between the inductor L1 and the battery, or electrically coupled between the inductor L1 and the rectified power source, or electrically coupled between the inductor L1 and the super capacitor.

[0054] Further, the DC load comprises a battery, a super capacitor, a resistor, a DC / DC converter or a DC / AC converter.

[0055] According to an embodiment of the present application, referring to Figures 2-3Each chopper further comprises a second capacitor, which is electrically coupled between the first end or the second end of the switch bridge arm and the second end of the inductor L1. Further, the first end of the switch bridge arm and the second end of the inductor L1 are coupled with a second capacitor Cf1, and the second end and the second end of the inductor L1 are respectively electrically coupled with a second capacitor Cf2.

[0056] Further, the rated working voltage of the first switch Q1 and the second switch Q2 is lower than the voltage of the DC element, and the voltage of the first capacitor CB1 is lower than the voltage of the DC element.

[0057] Referring to Figure 4 , the inductor L1 of the chopper is connected to the negative pole Bn- of the DC element, and the positive pole Bn+ of the DC element is connected together. Alternatively, referring to Figures 1-3 , the inductor L1 of the chopper is connected to the positive pole Bn+ of the DC element, and the negative pole Bn- of the DC element is connected together.

[0058] According to yet another embodiment of the present application, referring to Figure 5 , the power conversion system further comprises a compensation power supply, which is connected in parallel with the switch bridge arm. Specifically, the compensation power supply is connected to the local DC bus BUS.

[0059] According to yet another embodiment of the present application, referring to Figure 6 , one of the n DC elements B1-Bn is the DC end of the inverter 10. The remaining (n-1) are battery packs B1-Bn-1. Figure 6 is shown with n = 7. In some embodiments, the inverter 10 transmits power in one direction. In some embodiments, the inverter 10 can transmit power in both directions. In some embodiments, the inverter 10 is a PCS (power conditioning system). The power conversion system further comprises a control unit 11, which controls the first switch Q1 and the second switch Q2. The control unit 11 comprises a multiplexing controller 112 and a field controller 111. The multiplexing controller 112 is communicatively connected with the remaining (n-1) DC elements except the DC end of the inverter 10, and is electrically connected with the first switch Q1 and the second switch Q2 of each chopper respectively. The multiplexing controller 112 is connected with the inverter 10 through the field controller 111. Further, the voltage of the DC end of the inverter 10 is equal to the weighted average of the voltages of the remaining (n-1) DC elements. It can be understood that in the weighted average, the voltage of each of the remaining (n-1) DC elements is calculated with a weight equal to the ratio of the current flowing therethrough to the total current flowing through the remaining (n-1) DC elements.

[0060] In some embodiments, the remaining (n-1) DC elements are battery packs, and the control unit 11 controls the current flowing through the remaining (n-1) DC elements.

[0061] Specifically, referring to Figure 6 , a plurality of battery packs in a battery energy storage system are connected in parallel, and have the same rated voltage. For differences in SOC (state of charge) and SOH (state of health), the control unit 11 adjusts the charge and discharge current of the battery packs. Specifically, as shown in Figure 6 , n = 7, in which 6 choppers are connected to battery packs B1-B7 respectively, and the 7th chopper is connected to the DC port of the inverter 10. The voltage of the local DC bus BUS is much lower than the voltage of the battery packs B1-B7 and the DC port of the inverter 10. The multi-channel regulation controller 112 controls the current of each battery pack according to the SOC and SOH of each battery pack, and the 7th chopper controls the voltage of the DC port of the inverter 10, which is equal to the weighted average of the voltages of each battery pack. Because the voltage difference between the battery packs B1-B7 and the DC port of the inverter 10 is small, the voltage of the local DC bus BUS is low, and thus a very high system efficiency is obtained. For example, for a battery pack with a rated voltage of 1000V, the voltage difference between the unselected groups is 8%, i.e. the maximum voltage difference is 80V, and the voltage of the local DC bus BUS can be controlled to be 100V to 120V, which is much lower than the rated voltage of the battery pack.

[0062] In some embodiments, the voltage of the local DC bus BUS, i.e. the voltage of the first capacitor CB1, is controlled to a fixed value, which is lower than the voltage of the battery packs and the DC port of the inverter. In some embodiments, the fixed value is less than 50% of the rated voltage of the battery pack.

[0063] According to another embodiment of the present application, referring to Figure 7 , one of the n DC elements B1-Bn is the DC port of the inverter 10, and the remaining (n-1) are photovoltaic cell strings, such as the photovoltaic cell strings PV1, PV2 and PV3 shown in Figure 7 . Further, when the voltage of the DC port of the inverter 10 is near the average voltage of the maximum power point (MPP) voltages of the (n-1) photovoltaic cell strings, for example, within the range of 85% to 115% of the average voltage, the control unit controls the voltage at the second end of each inductor L1 with the MPP voltage of each photovoltaic cell string as the target value.

[0064] In particular, in the distributed PV system, the voltage at the DC side of the inverter 10 is controlled to be around the average of the MPP voltage of the PV strings, for example, 82% of the open circuit voltage of the PV strings. The difference between the voltage at the second end of the inductor L1 and the voltage at the DC side of the inverter 10 is regulated by the respective chopper, so that the voltage of each group of PV strings reaches the MPP operating point.

[0065] Referring to Figure 7 , n = 4, where the 3 choppers are connected to the PV strings B1, B2 and B3 respectively, and the 4th chopper is connected to the DC side of the inverter 10. The voltage across the switch bridge is much lower than the voltage across the PV strings B1, B2 and B3 and the DC side of the inverter 10. The voltage at the DC side of the inverter 10 is controlled by the inverter 10 to be close to 82% of the open circuit voltage of the PV strings. The difference between the voltage at the second end of the inductor L1 and the voltage at the DC side of the inverter 10 is regulated by the multi-regulation controller 112, so that the voltage of the PV strings tracks the MPP. Since the MPP voltage of the PV strings B1, B2 and B3 and the voltage at the DC side of the inverter 10 are relatively close, the voltage of the local DC bus BUS is relatively low, so that a very high system efficiency is achieved. Moreover, the power of each group of PV strings B1, B2 and B3 is compensated by the respective chopper, and is balanced by a compensation power source, for example, an isolated bidirectional or unidirectional DC-DC converter. For example, the voltage of the local DC bus BUS is controlled to be 50% of the operating voltage of the DC power source or load. Compared with the full voltage chopper, the switch device with a 50% lower operating voltage can be selected, and the converter loss is reduced under the same current condition.

[0066] Further, referring to Figure 8In some battery energy storage systems, the voltage range of the battery packs is slightly different from the voltage range of the DC side of the inverter 10. By using the multi-chopper and an additional compensation power supply, the charging and discharging current of each battery pack can be regulated by a low-voltage local DC bus BUS. Specifically, n = 4, where the 3-chopper is connected to the battery packs B1, B2 and B3, and the 4-chopper is connected to the DC side of the inverter 10. Assuming that the voltage of the DC side of the inverter 10 is slightly higher than the voltage of the battery packs B1, B2 and B3, the difference between the voltage of the DC side of the inverter 10 and the voltage of the battery packs B1, B2 and B3 is much smaller than the voltage of the battery packs B1, B2 and B3, and thus the voltage of the local DC bus BUS can be set to be slightly higher than the voltage difference. The control unit regulates the current flowing through the inductors L1 of the choppers to make the charging and discharging current of the battery packs B1, B2 and B3 meet the management requirements of the respective SOC and SOH. Since the voltage of the local DC bus BUS is low, the system efficiency is extremely high. The power sum of the charging and discharging circuits of the battery packs B1, B2 and B3 is compensated by the choppers and balanced through a compensation power supply, such as an isolated bidirectional DC-DC converter. For example, the voltage of the DC side of the inverter 10 is controlled to be 1000V, the voltage range of the battery packs B1, B2 and B3 is 700V to 920V, and the voltage of the local DC bus BUS is controlled to be 350V, which meets the requirement of the voltage increase of the battery packs B1, B2 and B3 when the voltage is at the lowest 700V and the highest 920V.

[0067] According to yet another aspect of the present application, there is also provided a control method for a power conversion system. Please refer to Figure 1 and Figure 9 simultaneously, the control method comprises:

[0068] Step S1, providing n choppers CH1-CHn, each of which comprises: a switch bridge arm comprising a first switch Q1 and a second switch Q2 connected in series; an inductor L1 having a first end connected to the midpoint of the switch bridge arm; and a first capacitor CB1 connected in parallel with the switch bridge arm, wherein the switch bridge arms of the choppers are connected in parallel;

[0069] Step S2, providing n DC elements B1-Bn corresponding to the n choppers CH1-CHn, the DC element being a DC power supply or a DC load, the first ends of the n DC elements B1-Bn being connected and the second ends being connected to the second ends of the inductors L1 of the corresponding choppers, at least one of the n DC elements B1-Bn being a DC power supply and at least one being a DC load, n being a natural number greater than or equal to 2; and

[0070] Step S3, regulating the current flowing through the DC element or the voltage of the DC element by controlling the first switch Q1 and the second switch Q2.

[0071] In some embodiments, the first switch Q1 and the second switch Q2 are controlled to operate in a complementary switching mode.

[0072] In the above embodiments, the choppers all employ a half-bridge structure. In this embodiment, to meet the high-voltage requirements, the chopper can adopt a three-level structure. For example... Figure 10 As shown, this embodiment is similar to Figure 1 The difference lies in that each switching arm includes a third switch Q3, a fourth switch Q4, a fifth switch Q5, and a sixth switch Q6 connected in series. The midpoint of the switching arm, i.e., the common connection point of the fourth switch Q4 and the fifth switch Q5, is connected to the first terminal of the inductor L1. Each chopper also includes a flying capacitor C1, which is electrically coupled between the common connection point of the third switch Q3 and the fourth switch Q4 and the common connection point of the fifth switch Q5 and the sixth switch Q6. Various modifications of the DC components and the control methods of the switching arms in the half-bridge structure embodiment are applicable to the three-level structure embodiment.

[0073] This invention employs a low-power non-isolated converter (n choppers) to replace a high-power converter, implementing current regulation and circulating current suppression for DC components. This invention can be used in energy storage systems with multiple battery packs connected in parallel, achieving low-cost circulating current suppression / current regulation / SOC optimization maintenance, improving battery life and safety, with a battery lifespan increase of approximately 15%.

[0074] Although the present invention has been disclosed above by way of embodiments, it is not intended to limit the present invention. Any person skilled in the art may make various modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope defined in the appended claims.

Claims

1. A power conversion system, characterized in that... include: n choppers, each chopper comprising: a switching bridge arm; an inductor, the first end of which is connected to the midpoint of the switching bridge arm; and a first capacitor connected in parallel with the switching bridge arm, wherein the switching bridge arms of each chopper are connected in parallel; and There are n DC components, each corresponding to one of the n choppers. Each DC component is a DC power supply or a DC load. The first end of each of the n DC components is connected but not directly electrically connected to the switching bridge arm of the n choppers. The second end of each component is connected to the second end of the inductor of the corresponding chopper. n is a natural number greater than or equal to 2.

2. The power conversion system according to claim 1, characterized in that, Of the n DC components, at least one is a DC power source and at least one is a DC load.

3. The power conversion system according to claim 1, characterized in that, The switch bridge arm includes a first switch and a second switch connected in series, and the common connection point of the first switch and the second switch is the midpoint of the switch bridge arm.

4. The power conversion system according to claim 1, characterized in that, The switch bridge arm includes a third switch, a fourth switch, a fifth switch, and a sixth switch connected in series. Each chopper also includes a flying capacitor electrically coupled between the common connection point of the third switch and the fourth switch and the common connection point of the fifth switch and the sixth switch. The common connection point of the fourth switch and the fifth switch is the midpoint of the switch bridge arm.

5. The power conversion system according to claim 1, characterized in that, The DC power source includes a battery, a rectifier, or a supercapacitor.

6. The power conversion system according to claim 5, characterized in that, The DC power supply also includes a DC-DC converter electrically coupled between the inductor and the battery, the rectifier, or the supercapacitor.

7. The power conversion system according to claim 1, 2, 5 or 6, characterized in that, The DC load includes a battery, a supercapacitor, a resistor, a DC / DC converter, or the DC terminal of a DC / AC converter.

8. The power conversion system according to any one of claims 1-6, characterized in that, Each chopper also includes a second capacitor electrically coupled between a first end and / or a second end of the switching bridge arm and a second end of the inductor.

9. The power conversion system according to any one of claims 1-6, characterized in that, The voltage of the first capacitor is lower than the voltage of the DC component.

10. The power conversion system according to any one of claims 1-6, characterized in that... It also includes a compensation power supply, which is connected in parallel with the switch bridge arm.

11. The power conversion system according to claim 1, characterized in that, One of the n DC components is the DC terminal of the inverter.

12. The power conversion system according to claim 11, characterized in that, It also includes a control unit for controlling the switch bridge arm.

13. The power conversion system according to claim 11, characterized in that, The voltage at the DC terminal of the inverter is equal to the weighted average of the voltages of the remaining (n-1) DC components.

14. The power conversion system according to claim 13, characterized in that, In the weighted average, the voltage calculation weight of each of the remaining (n-1) DC elements is the ratio of the current flowing through its DC element to the total current flowing through the remaining (n-1) DC elements.

15. The power conversion system according to claim 12, characterized in that, The remaining (n-1) DC components are battery packs, and the control unit controls the current flowing through the remaining (n-1) DC components.

16. The power conversion system according to claim 15, characterized in that, The voltage of the first capacitor is less than 50% of the rated voltage of the corresponding battery pack.

17. The power conversion system according to claim 12, characterized in that, The remaining (n-1) DC components are photovoltaic cell strings.

18. The power conversion system according to claim 17, characterized in that, When the voltage at the DC terminal of the inverter is near the average voltage of the maximum power point (MPP) voltages of (n-1) photovoltaic cell strings, the control unit controls the voltage at the second terminal of each inductor with the MPP voltage of each photovoltaic cell string as the target value.

19. A control method for a power conversion system, characterized in that, The control method includes the following steps: Provide n choppers, each chopper including: a switching bridge arm; an inductor, the first end of which is connected to the midpoint of the switching bridge arm; and a first capacitor connected in parallel with the switching bridge arm, wherein the switching bridge arms of each chopper are connected in parallel; Provide n DC components, each corresponding to one of the n choppers. These DC components are either DC power supplies or DC loads. The first terminals of each of the n DC components are connected but not directly electrically connected to the switching arms of the n choppers. The second terminals are respectively connected to the second terminals of the inductors of the corresponding choppers. n is a natural number greater than or equal to 2. The current flowing through the DC element or the voltage of the DC element can be adjusted by controlling the switch bridge arm.

20. The control method according to claim 19, characterized in that, One of the n DC components is the DC terminal of the inverter.

21. The control method according to claim 20, characterized in that, The voltage at the DC terminal of the inverter is controlled to be equal to the weighted average of the voltages of the remaining (n-1) DC components.

22. The control method according to claim 21, characterized in that, In the weighted average, the voltage of each of the remaining (n-1) DC elements is calculated as the ratio of the current flowing through it to the total current flowing through the remaining (n-1) DC elements.

23. The control method according to claim 20, characterized in that, The remaining (n-1) DC components are battery packs.

24. The control method according to claim 23, characterized in that, Control the voltage of the first capacitor to a fixed value.

25. The control method according to claim 24, characterized in that, This fixed value is lower than the voltage at the DC terminal of the battery pack and the inverter.

26. The control method according to claim 25, characterized in that, This fixed value is less than 50% of the rated voltage of the battery pack.

27. The control method according to claim 20, characterized in that... The remaining (n-1) DC components are photovoltaic cell strings.

28. The control method according to claim 27, characterized in that, The voltage at the DC terminal of the inverter is controlled to be close to the average voltage of the MPP voltage of (n-1) photovoltaic cell strings, and the voltage at the second terminal of each inductor is controlled with the MPP voltage of each photovoltaic cell string as the target value.

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