Patent Publication Number: US-2022231608-A1

Title: Power conversion system and method of controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 202110061168.3 filed in P.R. China on Jan. 18, 2021, the entire contents of which are hereby incorporated by reference. 
    
    
     Some references, if any, which may include patents, patent applications and various publications, may be cited and discussed in the description of this application. The citation and/or discussion of such references, if any, is provided merely to clarify the description of the present application and is not an admission that any such reference is “prior art” to the application described herein. All references listed, cited and/or discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference. 
     FIELD 
     The disclosure relates to the field of power conversion, and particularly to a power conversion system and a method of controlling the same. 
     BACKGROUND 
     A battery or supercapacitor for high power energy storage is often formed of tens of thousands of low voltage small cells, and high voltages and large currents desired by the system can be obtained after connection those small cells in series and parallel into multiple groups. However, imbalance of series-connected voltages and a circulating current of parallel-connected batteries restrict reliability and usability of the system, and seriously influence investing benefit or system safety. In recent two years, several fire accidents occurred at energy storage stations, reflecting urgent demand of the electric energy storage system for improving protection and safety performance of the system. For sake of safety, many energy storage systems have to limit a rang of SOC (state of charge), such as, from 20% to 80%, during charging and discharging of the batteries, in order to reserve a margin of 10% to 20% for a difference of voltages and a circulating current of the battery packs, causing an increase in cost of investment. 
     In order to solve the above problems, there are mainly two methods: 1. using bidirectional DC-DC converter (chopper) to achieve bidirectional conversion of voltages and currents of electric energy on both sides by taking each of the battery parks as an input and taking a DC bus of the power supplies or the loads as an output. The bidirectional DC-DC converters have some advantages, including regulating voltage and current, converting input voltage to ouput voltage, and rapid current switching performance, and some disadvantages, such as increasing cost and power loss of high voltage and large current DC conversion elements. 2. using series voltage compensation to regulate DC voltages and currents, and introducing two-port DC-DC converters for voltage compensation in a DC power supply system. These DC-DC converters for voltage compensation have input ports for supplying compensating electric energy, and output ports connected to power circuits of the power supply system for supplying voltage compensation. When compensation voltage is small relative to a power supply voltage, low power DC-DC converters for voltage compensation can be used to regulate a total output voltage and current. Since a power for compensation is largely decreased, as compared to full power input and full power output chopper, volume and cost of the conveters for compensation are reduced, and loss of power conversion is reduced. Such method is also referred to as partial power conversion. The DC-DC converters for compensation are isolated converters including high-frequency transformers, and there is a need to converter power from DC to high-frequency AC , and then to DC, so the number of elements is large, volume is large, and cost is high. 
     Therefore, it is quite necessary to find a power conversion system and a method of controlling the same, thereby solving one or more technical problems. 
     SUMMARY 
     In view of this, one object of the disclosure is to provide a power conversion system, which regulates voltages and/or currents of DC power supplies or loads, and further reduces the number and volume of elements to reduce cost and power loss by using low power non-isolated chopper with a specialized structure. 
     In order to achieve the object, according to one aspect of the disclosure, a power conversion system is provided, including n choppers, comprising n switching arms connected in parallel, each chopper comprising the switching arm, an inductor having a first end connected to a midpoint of the switching arm and a first capacitor connected in parallel to the switching arm; and 
     n DC components corresponding to the n choppers with a one-to-one relation, wherein one of the n DC components is a DC power supply or a DC load, and first ends of the n DC components are connected together, and a second end of each of the n DC components is connected to a second end of the inductor of the corresponding chopper, where n is a natural number greater than or equal to 2. 
     According to another aspect of the disclosure, wherein at least one of the n DC components is the DC power supply, and at least one of the n DC components is the DC load. 
     According to another aspect of the disclosure, wherein the switching arm includes a first switch and a second switch connected in series, and a common connection node of the first switch and the second switch is the midpoint of the switching arm. 
     According to another aspect of the disclosure, wherein the switching arm includes a third switch, a fourth switch, a fifth switch and a sixth switch connected in series, each of the choppers further includes a flying capacitor electrically coupled between a common connection node of the third switch and the fourth switch and a common connection node of the fifth switch and the sixth switch, and a common connection node of the fourth switch and the fifth switch is the midpoint of the switching arm. 
     According to another aspect of the disclosure, wherein the DC power supply includes battery, rectifier or supercapacitor. 
     According to another aspect of the disclosure, wherein the DC power supply further includes a DC-DC converter electrically coupled between the inductor and the battery, the rectifier or the supercapacitor. 
     According to another aspect of the disclosure, wherein the DC load comprises battery, supercapacitor, resistor, or a DC side of DC/DC converter or DC/AC converter. 
     According to another aspect of the disclosure, wherein each of the choppers further includes a second capacitor electrically coupled between a first end and/or a second end of the switching arm and the second end of the inductor. 
     According to another aspect of the disclosure, wherein a voltage of the first capacitor is lower than a voltage of the DC components. 
     According to another aspect of the disclosure, further including a compensation power supply connected in parallel to the switching arms. 
     According to another aspect of the disclosure, wherein one of the n DC components is a DC side of an inverter. 
     According to another aspect of the disclosure, further including a control unit for controlling the n switching arms. 
     According to another aspect of the disclosure, wherein a voltage at the DC side of the inverter is equal to a weighted mean value of voltages of the remaining (n−1) DC components. 
     According to another aspect of the disclosure, wherein a weight of the voltage of each of the remaining (n−1) DC components is calculated as a ratio of a current flowing through it to a total current flowing through the remaining (n−1) DC components. 
     According to another aspect of the disclosure, wherein the remaining (n−1) DC components are battery packs, and the control unit controls currents flowing through the remaining (n−1) DC components. 
     According to another aspect of the disclosure, wherein a voltage of the first capacitor is less than  50 % of a rated voltage of the corresponding battery pack. 
     According to another aspect of the disclosure, wherein the remaining (n−1) DC components are photovoltaic battery strings. 
     According to another aspect of the disclosure, wherein when a voltage at the DC side of the inverter is around an average voltage of the maximum power point (MPP) voltage of the (n−1) photovoltaic battery strings, the control unit controls a voltage at the second end of each of the inductors by using the MPP voltage of each of the photovoltaic battery strings as a target value. 
     According to another aspect of the disclosure, a method of controlling a power conversion system is further provided, including: 
     providing n choppers, the n choppers comprising n switching arms connected in parallel, each chopper comprising: the switching arm; an inductor having a first end connected to a midpoint of the switching arm; and a first capacitor connected in parallel to the switching arm; 
     providing n DC components corresponding to the n choppers with a one-to-one relation, wherein one of the n DC components is a DC power supply or a DC load, first ends of the n DC components are connected together, and a second end of each of the n DC components is connected to a second end of the inductor of the corresponding chopper, where n is a natural number greater than or equal to  2 ; and 
     regulating currents through the n DC components or voltages across the n DC components by controlling the n switching arms. 
     According to another aspect of the disclosure, wherein one of the n DC components is a DC side of an inverter. 
     According to another aspect of the disclosure, wherein a voltage at the DC side of the inverter is controlled to be equal to a weighted mean value of the voltages of the remaining (n−1) DC components. 
     According to another aspect of the disclosure, wherein a weight of the voltage of each of the remaining (n−1) DC components is calculated as a ratio of a current flowing through it to a total current flowing through the remaining (n−1) DC components. 
     According to another aspect of the disclosure, wherein the remaining (n−1) DC components are battery packs. 
     According to another aspect of the disclosure, wherein a voltage of the first capacitor is controlled to be a fixed value. 
     According to another aspect of the disclosure, wherein the fixed value is lower than a voltage of the battery packs and a voltage at the DC side of the inverter. 
     According to another aspect of the disclosure, wherein the fixed value is less than 50% of a rated voltage of the battery packs. 
     According to another aspect of the disclosure, wherein the remaining (n−1) DC components are photovoltaic battery strings. 
     According to another aspect of the disclosure, wherein a voltage at the DC side of the inverter is controlled to be around an average voltage of the MPP voltage of the (n−1) photovoltaic battery strings, and a voltage at the second end of each of the inductors is controlled by taking the MPP voltage of each of the photovoltaic battery strings as a target value. 
     The power conversion system of the disclosure replaces a large power converter with n choppers as low power non-isolated converters, to achieve current regulation and circulating current suppression of DC components such as the battery packs or supercapacitors. The disclosure can be applied to the energy storage system with multiple battery packs connected in parallel, achieves circulating current suppression/current regulation/SOC optimization and maintenance at low cost, and improves service life and safety of the batteries. The service life of the batteries is improved by about  15 %. 
     Hereinafter explanations are described in details with reference to the embodiments, and further interpretations are provided to the technical solution of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To make the above and other objects, features, advantages and examples of the disclosure more apparent, the accompanying drawings are explained as follows: 
         FIG. 1  is a circuit diagram of a power conversion system according to a first embodiment of the disclosure. 
         FIG. 2  is a circuit diagram of a power conversion system having multiple types of DC power supplies or DC loads according to a second embodiment of the disclosure. 
         FIG. 3  is a circuit diagram of a power conversion system configured with filter capacitors at different positions according to a third embodiment of the disclosure. 
         FIG. 4  is a circuit diagram of a power conversion system according to a fourth embodiment of the disclosure, in which inductors of choppers are connected to negative electrodes of DC power supplies or loads, and positive electrodes of the power supplies or loads are connected together in parallel. 
         FIG. 5  is a circuit diagram of a power conversion system according to a fifth embodiment of the disclosure, in which a local DC bus is connected to a compensation power supply. 
         FIG. 6  is a circuit diagram of a power conversion system according to a sixth embodiment of the disclosure. 
         FIG. 7  is a circuit diagram of a power conversion system according to a seventh embodiment of the disclosure. 
         FIG. 8  is a circuit diagram of a power conversion system according to an eighth embodiment of the disclosure. 
         FIG. 9  is a flow chart of a method of controlling a power conversion system according to one embodiment of the disclosure. 
         FIG. 10  is a circuit diagram of a power conversion system according to a ninth embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     To make description of the disclosure more specific and complete, the accompanying drawings and various examples can be referred, and the same numbers in the drawings represent the same or similar components. On the other hand, the commonly known components and steps are not described in the examples to avoid unnecessary limit to the disclosure. In addition, for sake of simplifying the drawings, some known common structures and elements are illustrated in the drawings in a simple manner. 
     According to one embodiment of the disclosure, referring to  FIG. 1 , a power conversion system is provided, including n choppers CH 1 -CHn and n DC components B 1 -Bn, where n is a natural number greater than or equal to 2. The n choppers CH 1 -CHn includes n switching arms connected in parallel. Each chopper includes: the switching arm, an inductor L  1  having a first end connected to a midpoint of the switching arm, and a first capacitor CB 1  connected in parallel to the switching arm. Further, each switching arm includes a first switch Q 1  and a second switch Q 2  connected in parallel. The inductor L 1  has a first end connected to a midpoint of the switching arm, i.e., a common connection node of the first switch Q 1  and the second switch Q 2 . The first capacitor CB 1  is connected in parallel to the switching arm. The n DC components B 1 -Bn correspond to the n choppers CH 1 -CHn with a one-to-one relation, and the DC component is a DC power supply or a DC load. First ends of the n DC components are connected together, and a second end of each the n DC components is connected to a second end of the inductor L 1  of the corresponding chopper. At least one of the n DC components B  1 -Bn is a DC power supply, and at least one of the n DC components B 1 -Bn is a DC load. In some embodiments, the choppers work unidirectionally. In some embodiments, the choppers work bidirectionally. In some embodiments, a part of the choppers work unidirectionally, and a part of the choppers work bidirectionally. Further, each switching arm is connected in parallel between two ends LB+and LB- of a local DC bus L. There is no direct connection between the DC components B 1 -Bn and the first capacitor CB 1 , i.e., the local DC bus L. 
     Specifically, the DC power supply includes battery, rectifier or supercapacitor. The DC power supply may further include DC-DC converter electrically coupled between the inductors L 1  and the battery, or electrically coupled between the inductors L 1  and the rectifier, or electrically coupled between the inductors L 1  and the supercapacitor. 
     Further, the DC load include battery, supercapacitor, resistor, or DC side of DC/DC converter or DC/AC converter. 
     Referring to  FIGS. 2-3 , each of the choppers further includes a second capacitor electrically coupled between a first end or a second end of the switching arm and the second end of the inductor L 1 . Further, the second capacitor Cfl is coupled between the first end of the switching arm and the second end of the inductor L 1 , and the second capacitor Cf 2  is electrically coupled between the second end of the switching arm and the second end of the inductor L 1 . 
     Further, the rated working voltages of the first switch Q 1  and the second switch Q 2  are lower than a voltage of the DC components, and a voltage of the first capacitor CB 1  is lower than the voltage of the DC components. 
     Referring to  FIG. 4 , the inductors L 1  of the choppers are connected to negative electrodes Bn- of the DC components, and positive electrodes Bn+ of the DC components are connected together in parallel. Alternatively, referring to  FIGS. 1-3 , the inductors L 1  of the choppers are connected to positive electrodes Bn+ of the DC components, and negative electrodes Bn- of the DC components are connected together. 
     According to another embodiment of the disclosure, referring to  FIG. 5 , the power conversion system further includes a compensation power supply connected in parallel to the switching arm. Specifically, the compensation power supply is connected to the local DC bus L. 
     According to another embodiment of the disclosure, referring to  FIG. 6 , one of the n DC components B 1 -Bn is a DC side of an inverter  10 , and the remaining (n−1) DC components are battery packs B 1 -Bn- 1 .  FIG. 6  illustrates a case in which n=7. In some embodiments, the inverter  10  transmits power unidirectionally. In some embodiments, the inverter  10  may transmit power bidirectionally. In some embodiments, the inverter  10  is a power conditioning system (PCS). The power conversion system further includes a control unit  11  for controlling the first switch Q 1  and the second switch Q 2 . The control unit  11  includes a multipath regulating controller  112  and a field controller  111 . The multipath regulating controller  112  is in communication with the remaining (n−1) DC components, except the DC side of the inverter  10 , and is electrically connected to the first switch Q 1  and the second switch Q 2  of each chopper. The multipath regulating controller  112  is connected to the inverter  10  through the field controller  111 . Further, a voltage at the DC side of the inverter  10  is equal to a weighted mean value of a voltage of the remaining (n−1) DC components. It can be understood that a weight of the voltage of each of the remaining (n−1) DC components is calculated as a ratio of a current flowing through the DC component to a total current flowing through the remaining (n−1) DC components. 
     In some embodiments, the remaining (n−1) DC components are battery packs, and the control unit  11  controls currents flowing through the remaining (n−1) DC components. 
     Specifically, referring to  FIG. 6 , a plurality of battery packs in the battery energy storage system are connected in parallel, and their rated voltages are the same. With respect to a difference in for example SOC (state of charge)/SOH (state of health), the control unit  11  regulates charging and discharging currents of the battery packs. Specifically, as shown in  FIG. 6 , n=7, where 6 choppers are connected to the battery packs B 1 -B 7 , respectively, and the 7 th  chopper is connected to the DC side of the inverter  10 . A voltage of the local DC bus L is far less than a voltage of the battery packs B 1 -B 7  or a voltage at the DC side of the inverter  10 . The multipath regulating controller  112  controls currents of the respective batteries depending on SOC and SOH of the respective battery packs, the 7 th  chopper controls the voltage at the DC side of the inverter  10 , and the voltage at the DC side of the inverter  10  is equal to a weighted mean value of voltages of all battery packs. 
     Since a difference between the voltage of the battery packs B 1 -B 7  and the voltage at the DC side of the inverter  10  is small, the voltage of the local DC bus L is low, so an extremely high efficiency of the system is obtained. For example, for battery packs with a rated voltage of 1000V, a voltage difference of 8% exists between the unscreened groups, i.e., the maximum voltage 80V, and the voltage of the local DC bus L can be controlled from 100V to 120V, which is far less than the rated voltage of the battery pack. 
     In some embodiments, the voltage of the local DC bus L, i.e., the voltage of the first capacitor CB 1 , is controlled to be a fixed value, and the fixed voltage is lower than a voltage of the battery packs and a voltage at the DC side of the inverter. In some embodiments, the fixed value is less than 50% of a rated voltage of the battery packs. 
     According to another embodiment of the disclosure, referring to  FIG. 7 , one of the n DC components B 1 -Bn is a DC side of an inverter  10 , and the remaining (n−1) DC components are photovoltaic battery strings PV 1 , PV 2  and PV 3 , as shown in  FIG. 7 . Further, when a voltage at the DC side of the inverter  10  is around an average voltage of the maximum power point (MPP) voltage of the (n−1) photovoltaic battery strings, for example, within a range of 85% to 115% of the average voltage, the control unit controls a voltage at the second end of each of the inductors L 1  by taking the MPP voltage of each of the photovoltaic battery strings as a target value. 
     Specifically, in a distributed PV power generation system, the inverter  10  controls the voltage at the DC side of the inverter to be around an average value of the MPP voltage of the multipath photovoltaic battery strings, for example, about 82% of an open-circuit voltage of the photovoltaic battery strings, and each of the choppers regulate a difference between the voltage at the second end of the inductor L 1  and the voltage at the DC side of the inverter  10 , such that a voltage of each photovoltaic battery string can reach the MPP working point. 
     Referring to  FIG. 7 , n=4, where three choppers are connected to the photovoltaic battery strings B 1 , B 2  and B 3 , respectively, and the  4   th  chopper is connected to the DC side of the inverter  10 . A voltage across the switching arm is far less than the voltage of the photovoltaic battery strings B 1 , B 2  and B 3  or the voltage at the DC side of the inverter  10 , and the inverter  10  controls the voltage at the DC side of the inverter to be about  0 . 82  of an open-circuit voltage of the photovoltaic battery strings. The multipath regulating controller  112  regulates a difference between the voltage at the second end of the inductor L 1  of each chopper and the voltage at the DC side of the inverter  10 , such that the voltage of the photovoltaic battery strings tracks the MPP. Since a difference between the MPP voltage of the photovoltaic battery strings B 1 , B 2  and B 3  and the voltage at the DC side of the inverter  10  is small, the voltage of the local DC bus Lis low, so an extremely high efficiency of the system is obtained. For example, the voltage of the local DC bus is controlled to be 50% of a working voltage of the DC power supply or load, and as compared to full voltage choppers, switching devices fit to dc bus voltage can be used, and loss of the choppers is reduced in the case of the same current. Moreover, the total power from the choppers compensated into photovoltaic cell string B 1 , B 2 , and B 3 , can be supplied through one compensation power supply, for example the isolated bidirectional or unidirectional DC-DC converter. 
     Further, referring to  FIG. 8 , in some battery energy storage systems, there is small difference between a voltage range of the battery packs and a voltage range of the DC side of the inverter  10 . By using multipath choppers and the additional compensation power supply, charging and discharging currents of each group of batteries can be regulated through the local DC bus L with a low voltage. Specifically, in a case that n=4, where three choppers are connected to the battery packs B  1 , B 2  and B 3 , respectively, and the 4 th  chopper is connected to the DC side of the inverter  10 . Assuming that the voltage at the DC side of the inverter  10  is slightly higher than the voltage of the battery packs B 1 , B 2  and B 3 , a difference between the voltage at the DC side of the inverter  10  and the voltage of the battery packs B 1 , B 2  and B 3  is far less than the voltage of the battery packs B 1 , B 2  and B 3 , so the voltage of the local DC bus L can be set to be slightly higher than the voltage difference. The control unit regulates currents of the inductors L 1  of the respective choppers, such that the charging and discharging currents of the battery packs B 1 , B 2  and B 3  conform to management requirements of the SOC and SOH. Since the voltage of the local DC bus L is low, an extremely high efficiency of the system is obtained. For example, the voltage at the DC side of the inverter  10  is controlled to be 1000V, the voltage range of the battery packs B 1 , B 2  and B 3  is from 720V to 920V, and the voltage of the local DC bus L is controlled to be 350V, satisfying the requirement for improvement of the voltage of the battery packs B 1 , B 2  and B 3  when the minimum voltage is 700V and the maximum voltage is 920V. The total power from the choppers compensated into photovoltaic cell string B 1 , B 2 , and B 3 , can be supplied through one compensation power supply, for example the isolated bidirectional or unidirectional DC-DC converter. 
     According to another aspect of the disclosure, a method of controlling a power conversion system is further improved. Please refer to  FIGS. 1 and 9 , the control method includes: 
     In step S 1 , providing n choppers CH 1 -CHn, each including: a switching arm including a first switch Q 1  and a second switch Q 2  connected in series, an inductor L 1  having a first end connected to a midpoint of the switching arm, and a first capacitor CB 1  connected in parallel to the switching arm, wherein the switching arms of all the choppers are connected in parallel. 
     In step S 2 , providing n DC components B 1 -Bn corresponding to the n choppers CH 1 -CHn with a one-to-one relation, wherein the DC component is DC power supply or DC load. First ends of the n DC components B 1 -Bn are connected together, and second ends of the n DC components B 1 -Bn are connected to second ends of the inductors L 1  of the corresponding choppers respectively, at least one of the n DC components B 1 -Bn is a DC power supply, and at least one is a DC load, where n is a natural number greater than or equal to 2. 
     In step S 3 , regulating current through the DC component or voltage across the DC component by controlling the first switch Q 1  and the second switch Q 2 . 
     In some embodiments, the first switch Q 1  and the second switch Q 2  are controlled to work in a complementary conduction manner. 
     The choppers in the above embodiments all use a half-bridge structure. In this embodiment, in order to satisfy the requirement for a high voltage, the choppers can use a three-level structure. As shown in  FIG. 10 , a difference between this embodiment and  FIG. 1  lies in that each switching arm includes a third switch Q 3 , a fourth switch Q 4 , a fifth switch Q 5  and a sixth switch Q 6  connected in series. The midpoint of the switching arm, i.e., a common connection node of the fourth switch Q 4  and the fifth switch Q 5 , is connected to the first end of the inductor L 1 . Each chopper further includes a flying capacitor C 1  electrically coupled between a common connection node of the third switch Q 3  and the fourth switch Q 4  and a common connection node of the fifth switch Q 5  and the sixth switch Q 6 . Various deformations of the DC components and the control method of the switching arm in this embodiment using the half-bridge structure are applicable to the embodiment using the three-level structure. 
     The disclosure replaces a large power converter with low power non-isolated converters (n choppers), to achieve current regulation and circulating current suppression of the DC components. The disclosure can be applied to the energy storage system with multiple battery packs connected in parallel, achieves circulating current suppression/current regulation/SOC optimization and maintenance at low cost, and improves service life and safety of the batteries. The service life of the batteries is improved by about 15%. 
     Although the disclosure has been disclosed in the embodiments, the disclosure is not limited thereto. Any skilled in the art shall make various variations and modifications without departing from spirit and scope of the disclosure, so the protection scope of the disclosure shall be subjected to the scope defined by the appended claims.