Current control circuit and control method for parallel DC voltage sources

Abstract

This invention discloses a current control circuit and method for parallel DC voltage sources. The circuit includes a low-voltage DC bus capacitor connected in parallel to the output terminal of a low-voltage DC power supply. Multiple power electronic half-bridge units are connected in parallel between the positive and negative terminals of the low-voltage DC bus. The neutral point output of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to form a current-sharing branch. The same terminal of the multiple DC voltage sources is connected to multiple current-sharing branches of the current control circuit. The other terminals of the multiple DC voltage sources are connected together as one output terminal of a medium-voltage DC bus. Simultaneously, one terminal of the low-voltage DC bus serves as the other output terminal of the medium-voltage DC bus. A filter capacitor is connected in parallel between the positive and negative terminals of the medium-voltage DC bus, which then serves as the output terminal of the parallel DC voltage sources to supply power to the load. This invention can convert parallel DC voltage sources into parallel current sources, significantly reducing the cost and losses of the DC / DC converter.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a current control circuit and control method for parallel DC voltage sources. Background Technology

[0002] In battery energy storage, to avoid parallel circulating current issues between battery clusters, a string structure is typically used. Each battery cluster is boosted by a full-power DC / DC converter before being connected in parallel, effectively controlling the charging and discharging current and output voltage of each cluster. In photovoltaic (PV) power generation, to achieve maximum power point tracking (MPPT) for PV panels, a common approach is to connect a full-power DC / DC converter to the output of each PV string. The DC / DC converter controls the output voltage and current of the PV string, thus achieving MPPT. These scenarios essentially involve current control problems with parallel DC voltage sources. While adding a full-power DC / DC converter can improve control performance, it also introduces drawbacks such as increased cost and reduced system efficiency. Improving the performance and efficiency of string energy storage and PV systems while reducing costs is a pressing challenge for the PV and energy storage industries. A schematic diagram of a string PV or energy storage system provided by existing technology is shown below. Figure 1 As shown, each photovoltaic (PV) or energy storage branch needs to be regulated by a full-power DC / DC converter before being connected in parallel. This DC / DC converter can ensure stable output voltage and adapt to voltage variations in different PV or energy storage branches, while also controlling the output current of each branch. However, this DC / DC converter needs to operate under both high voltage and high current conditions, resulting in significant losses and high costs. Summary of the Invention

[0003] The present invention aims to at least partially solve one of the technical problems in the related art.

[0004] To avoid the uneven current problem caused by direct parallel connection of DC voltage sources such as photovoltaic or energy storage systems, and to reduce the power and cost of DC / DC converters in string photovoltaic or energy storage systems, this invention proposes a current control circuit and method for parallel DC voltage sources. The idea is to first connect an inductor in series with each DC voltage source branch to become a current source, and then connect a power electronic half-bridge unit in series. The current of the branch inductor is adjusted by PWM chopping of the power electronic half-bridge unit, thereby achieving precise control of the current of each DC voltage source.

[0005] Another objective of this invention is to provide a control method for a current control circuit with parallel DC voltage sources.

[0006] To achieve the above objectives, this invention proposes a current control circuit for parallel DC voltage sources, comprising a low-voltage DC power supply, a low-voltage DC bus capacitor, multiple power electronic half-bridge units, multiple branch inductors, and multiple DC switches. The output terminal of the low-voltage DC power supply is connected in parallel to the low-voltage DC bus capacitor. The multiple power electronic half-bridge units are connected in parallel between the positive and negative terminals of the low-voltage DC bus. The neutral point output of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to form a current-sharing branch. The same terminal of the multiple DC voltage sources is connected to the multiple current-sharing branches of the current control circuit, and the other terminals of the multiple DC voltage sources are connected together to form one output terminal of the medium-voltage DC bus. At the same time, one terminal of the low-voltage DC bus is used as the other output terminal of the medium-voltage DC bus. A filter capacitor is connected in parallel between the positive and negative terminals of the medium-voltage DC bus to provide power to the load as the output terminal of the parallel DC voltage sources.

[0007] The current control circuit of the parallel DC voltage source in this embodiment of the invention may also have the following additional technical features:

[0008] In one embodiment of the present invention, the positive terminals of multiple DC voltage sources are respectively connected to the current sharing branch, and the negative terminals of multiple DC voltage sources are connected together as the negative terminal of the medium-voltage DC bus. At the same time, the positive terminal of the low-voltage DC bus is used as the positive terminal of the medium-voltage DC bus, or the negative terminal of the low-voltage DC bus is used as the positive terminal of the medium-voltage DC bus.

[0009] In one embodiment of the present invention, the negative terminals of multiple DC voltage sources are respectively connected to the current sharing branch, and the positive terminals of multiple DC voltage sources are connected together as the positive terminal of the medium-voltage DC bus. At the same time, the negative terminal of the low-voltage DC bus is used as the negative terminal of the medium-voltage DC bus, or the positive terminal of the low-voltage DC bus is used as the negative terminal of the medium-voltage DC bus.

[0010] In one embodiment of the present invention, it is assumed that the voltages of the plurality of DC voltage sources are U1, U2, ..., U... n The highest voltage is U. max The lowest voltage is U min The low-voltage DC bus voltage is U b The medium-voltage DC bus voltage is U d The power electronic half-bridge unit operates in a complementary state, and the upper transistor T of the power electronic half-bridge unit is defined. j The switching function is S j j = 1, 2, ..., n, when the upper pipe is on and the lower pipe is off, S j =1, when the lower tube is on and the upper tube is off, S j =0.

[0011] In one embodiment of the present invention, according to the first circuit connection method of the current control circuit, with the current flowing out of the half-bridge unit as the positive direction, the inductor voltage on the j-th branch is:

[0012] u Lj =U d -U j -(1-S j )U b (1)

[0013] When the half-bridge unit operates in PWM chopper mode, equation (1) satisfies the condition in S. j =1 when u Lj >0, the inductor current increases, in S j =0 when u Lj <0, the inductor current decreases, making the average inductor current controllable. The output voltage of the medium-voltage DC bus must meet the following requirements:

[0014] U max d b +U min (2)

[0015] Based on the fact that the increase and decrease of the inductor current are equal during a switching cycle in steady state, according to equation (1):

[0016]

[0017] Where T on and T off The upper tube T of the half-bridge unit j The time L is the time for turning on and off within a switching cycle. j Given the branch inductance, the steady-state switching function S is obtained. j The duty cycle in one switching cycle is:

[0018]

[0019] According to the second circuit connection method of the current control circuit, the inductor voltage on the j-th branch is:

[0020] u Lj =U d -U j +S j ·U b (5)

[0021] The output voltage of the medium-voltage DC bus must meet the following requirements:

[0022] U max -U b d min (6)​​​​

[0023] steady-state switching function S j The duty cycle in one switching cycle is:

[0024] d j =(U j -U d ) / U b (7)

[0025] According to the third circuit connection method of the current control circuit, the inductor voltage on the j-th branch is:

[0026] u Lj =U j +S j ·U b -U d (8)

[0027] The output voltage of the medium-voltage DC bus must meet the following requirements:

[0028] U max d b +U min (9)

[0029] steady-state switching function S j The duty cycle in one switching cycle is:

[0030] d j =(U d -U j ) / U b (10)

[0031] According to the fourth circuit connection method of the current control circuit, the inductor voltage on the j-th branch is:

[0032] u Lj =U j -(1-S j )·U b -U d (11)

[0033] The output voltage of the medium-voltage DC bus must meet the following requirements:

[0034] U max -U b d min (12)

[0035] steady-state switching function S j The duty cycle in one switching cycle is:

[0036] d j =(U​​​​d +U b -U j ) / U b (13)

[0037] In one embodiment of the present invention, when the DC voltage source is a photovoltaic string and a power supply requiring unidirectional power output is needed, one of the two fully controlled switching devices of the power electronic half-bridge unit is replaced with a diode.

[0038] In one embodiment of the present invention, when the positive terminal of the DC voltage source is connected to the current sharing branch, the upper transistor of the power electronic half-bridge unit is replaced with a diode; when the negative terminal of the DC voltage source is connected to the current sharing branch, the lower transistor of the power electronic half-bridge unit is simplified to a diode.

[0039] In one embodiment of the present invention, the low-voltage DC power supply is an isolated AC / DC converter powered by an AC power source, or an isolated DC / DC converter powered by a DC power source.

[0040] To achieve the above objectives, another aspect of the present invention proposes a control method for a current control circuit of a DC voltage source in parallel. For the DC voltage source with controlled charging and discharging power, a dual closed-loop control of voltage and current is adopted. The method includes:

[0041] Based on different circuit connection methods, the allowable operating voltage range of the DC voltage source is determined according to formulas (2), (6), (9) or (12) based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply.

[0042] Obtain the operating voltage of the DC voltage source. If it exceeds the preset operating voltage range, disconnect the DC switch of the corresponding DC voltage source branch.

[0043] The total current setpoint for all branches is obtained by performing closed-loop control on the output voltage of the medium-voltage DC bus.

[0044] The current setpoint for each branch is assigned according to the voltage level of the DC voltage source of each branch or the control command.

[0045] The inductor current of each branch is controlled in a closed loop according to the assigned current setpoint of each branch, thereby obtaining the switching control signal of the power electronic half-bridge unit of each branch.

[0046] The current control circuit and method for parallel DC voltage sources in this invention can realize the current control circuit and control of DC voltage sources such as battery clusters and photovoltaic strings that are not directly connected in parallel. By connecting a low-voltage, low-power current source control circuit in series with each DC voltage source, the parallel connection of DC voltage sources can be transformed into the parallel connection of current sources. The current control is simple and highly accurate. Compared with the traditional string structure, it can also significantly reduce the cost and loss of DC / DC converters.

[0047] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0048] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0049] Figure 1 It is a parallel circuit of a string voltage source in existing technology;

[0050] Figure 2 This is a structural diagram of the DC voltage source parallel current control circuit according to an embodiment of the present invention;

[0051] Figure 3 This is a schematic diagram of the first connection method of the DC voltage source parallel current control circuit according to an embodiment of the present invention;

[0052] Figure 4 This is a schematic diagram of the second connection method of the DC voltage source parallel current control circuit according to an embodiment of the present invention;

[0053] Figure 5 This is a schematic diagram of the third connection method of the DC voltage source parallel current control circuit according to an embodiment of the present invention;

[0054] Figure 6 This is a schematic diagram of the fourth connection method of the DC voltage source parallel current control circuit according to an embodiment of the present invention;

[0055] Figure 7 This is a schematic diagram of the first connection method of the DC voltage source parallel current control circuit for photovoltaic strings according to an embodiment of the present invention;

[0056] Figure 8 This is a schematic diagram of a second connection method of a DC voltage source parallel current control circuit for photovoltaic strings according to an embodiment of the present invention;

[0057] Figure 9 This is a schematic diagram of the third connection method of the DC voltage source parallel current control circuit for photovoltaic strings according to an embodiment of the present invention;

[0058] Figure 10This is a schematic diagram of the fourth connection method of the DC voltage source parallel current control circuit for photovoltaic strings according to an embodiment of the present invention;

[0059] Figure 11 This is a flowchart of a control method for a current control circuit with parallel DC voltage sources according to an embodiment of the present invention. Detailed Implementation

[0060] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0061] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0062] The following description, with reference to the accompanying drawings, describes a current control circuit and control method for parallel DC voltage sources according to an embodiment of the present invention.

[0063] Figure 2 This is a flowchart of a current control circuit for a parallel DC voltage source according to an embodiment of the present invention. Figure 2 As shown, it includes a low-voltage DC power supply and a low-voltage DC bus capacitor C. b There are n power electronic half-bridge units (n is a positive integer), n branch inductors, and n DC switches. A low-voltage DC bus capacitor C is connected in parallel to the output of the low-voltage DC power supply. b To provide a stable DC voltage, all power electronic half-bridge units are connected in parallel between the positive (LVDC+) and negative (LVDC-) terminals of the low-voltage DC bus. The neutral point output of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to form a current-sharing branch. To achieve output current control of the n parallel DC voltage sources, the same terminal of each of the n DC voltage sources is connected to one of the n current-sharing branches of the current control circuit. The other terminals of the n DC voltage sources are connected together as one output terminal of the medium-voltage DC bus. Simultaneously, one terminal of the low-voltage DC bus is used as the other output terminal of the medium-voltage DC bus. A filter capacitor is connected in parallel between the positive and negative terminals of the medium-voltage DC bus to serve as the output terminal of the parallel DC voltage sources, supplying power to the load.

[0064] Preferably, the positive terminals of the n DC voltage sources can be connected to the current-sharing branch respectively, such as... Figure 3 and Figure 4As shown, at this time, the negative terminals of n DC voltage sources are connected together as the negative terminal DC- of the medium-voltage DC bus, while the positive terminal of the low-voltage DC bus is used as the positive terminal DC+ of the medium-voltage DC bus, as follows. Figure 3 As shown, or the negative terminal of the low-voltage DC bus can be used as the positive terminal DC+ of the medium-voltage DC bus, such as... Figure 4 As shown.

[0065] Preferably, the negative terminals of the n DC voltage sources can also be connected to the current-sharing branch respectively, such as... Figure 5 and Figure 6 As shown, the positive terminals of the n DC voltage sources are connected together as the positive terminal DC+ of the medium-voltage DC bus, while the negative terminal of the low-voltage DC bus is used as the negative terminal DC- of the medium-voltage DC bus. Figure 5 As shown, or the positive terminal of the low-voltage DC bus can be used as the negative terminal DC- of the medium-voltage DC bus, such as... Figure 6 As shown.

[0066] In one embodiment of the present invention, it is assumed that the voltages of the plurality of DC voltage sources are U1, U2, ..., U... n The highest voltage is U. max The lowest voltage is U min The low-voltage DC bus voltage is U b The medium-voltage DC bus voltage is U d The power electronic half-bridge unit operates in a complementary state. The upper transistor T of the power electronic half-bridge unit is defined as follows: j The switching function is S j j = 1, 2, ..., n, when the upper pipe is on and the lower pipe is off, S j =1, when the lower tube is on and the upper tube is off, S j =0.

[0067] In one embodiment of the present invention, Figure 3 Taking the connection shown as an example, with the current flowing out of the half-bridge unit as the positive direction, the inductor voltage on the j-th branch can be written as:

[0068] u Lj =U d -U j -(1-S j )U b (1)

[0069] To make the inductor current controllable, when the half-bridge unit operates in PWM chopper mode, equation (1) must satisfy the condition that the inductor current is controllable in S. j =1 when u Lj >0, the inductor current increases, in S j =0 when u Lj When the value is less than 0, the inductor current decreases, making the average value of the inductor current controllable. Therefore, the output voltage of the medium-voltage DC bus must meet the following requirements:

[0070] U max d b +U min (2)

[0071] From equation (2), we can derive the output voltage U of the medium-voltage DC bus. d It must be greater than the highest voltage of the DC power supply and less than the sum of the lowest voltage of the DC power supply and the low-voltage DC bus voltage. Therefore, the low-voltage DC bus voltage U b It must also be greater than the highest voltage U of the DC power supply. max With the lowest voltage U min difference.

[0072] Since the increase and decrease of the inductor current are equal during one switching cycle in steady state, we can derive the following from equation (1):

[0073]

[0074] Where T on and T off The upper tube T of the half-bridge unit j The time L is the time for turning on and off within a switching cycle. j Given the branch inductance, the steady-state switching function S can be derived. j The duty cycle in one switching cycle is:

[0075]

[0076] Substituting equation (2) into equation (4), we obtain the duty cycle d. j The range is between 0 and 1, proving the correctness of the analysis results.

[0077] In one embodiment of the present invention, for Figure 4 With the connection shown, the inductor voltage on the j-th branch can be written as:

[0078] u Lj =U d -U j +S j ·U b (5)

[0079] Following the same logic, it can be deduced that the output voltage of the medium-voltage DC bus must meet the following requirements:

[0080] U max -U b d min (6)

[0081] steady-state switching function S j ​​​​The duty cycle in one switching cycle is:

[0082] d j =(U j -U d ) / U b (7)

[0083] Substituting equation (6) into equation (7), we obtain the duty cycle d. j The range is between 0 and 1, proving the correctness of the analysis results.

[0084] In one embodiment of the present invention, for Figure 5 With the connection shown, the inductor voltage on the j-th branch can be written as:

[0085] u Lj =U j +S j ·U b -U d (8)

[0086] Following the same logic, it can be deduced that the output voltage of the medium-voltage DC bus must meet the following requirements:

[0087] U max d b +U min (9)

[0088] At this point, the steady-state switching function S j The duty cycle in one switching cycle is:

[0089] d j =(U d -U j ) / U b (10)

[0090] Substituting equation (9) into equation (10), we obtain the duty cycle d. j The range is between 0 and 1, proving the correctness of the analysis results.

[0091] In one embodiment of the present invention, for Figure 6 With the connection shown, the inductor voltage on the j-th branch can be written as:

[0092] u Lj =U j -(1-S j )·U b -U d (11)

[0093] Following the same logic, it can be deduced that the output voltage of the medium-voltage DC bus must meet the following requirements:

[0094] U​​max -U b d min (12)

[0095] steady-state switching function S j The duty cycle in one switching cycle is:

[0096] d j =(U d +U b -U j ) / U b (13)

[0097] Substituting equation (12) into equation (13), we can obtain the duty cycle d. j The range is between 0 and 1, proving the correctness of the analysis results.

[0098] Therefore, for the above four different connection methods, as long as the magnitude relationship between the medium-voltage DC bus voltage, the low-voltage DC bus voltage, and the DC voltage source voltage satisfies equations (2), (6), (9), or (12) respectively, the inductor current can be controlled by controlling the duty cycle of the half-bridge unit switch. In steady state, its duty cycle satisfies equations (4), (7), (10), or (13) respectively. If the DC voltage source voltage of a certain branch is too high or too low, causing it to fail to satisfy equations (2), (6), (9), or (12), the DC switch of that branch can be disconnected to cut off that branch.

[0099] Furthermore, when the DC power supply is a photovoltaic string or similar power source that only requires unidirectional power output, one of the two fully controlled switching devices in the power electronic half-bridge unit can be simplified to a diode to reduce costs. When the positive terminal of the DC voltage source is connected to the current-sharing branch, since the current always flows from the photovoltaic string into the current-sharing branch, such as... Figure 7 and Figure 8 As shown, the upper transistor of the power electronic half-bridge unit can be simplified to a diode; when the negative terminal of the DC voltage source is connected to the current sharing branch, since the current always flows into the photovoltaic string from the current sharing branch, as shown... Figure 9 and Figure 10 As shown, the lower transistor of the power electronic half-bridge unit can be simplified to a diode.

[0100] Furthermore, the low-voltage DC power supply is an isolated AC / DC converter powered by an AC power source, or an isolated DC / DC converter powered by a DC power source.

[0101] ​​The current control circuit for parallel DC voltage sources according to embodiments of the present invention converts the parallel connection of DC voltage sources into parallel connection of current sources by connecting an inductor and a power electronic half-bridge unit in series to each DC voltage source branch. It adopts current closed-loop control with high accuracy and good current control performance. The proposed current control circuit has low voltage and low power, avoiding the need for a full-power DC / DC converter in traditional string schemes, which greatly reduces the system size and cost and improves efficiency. The proposed current control circuit adopts a common DC bus structure, and all branches only need to be powered by a common independent DC power supply, which reduces cost.

[0102] To achieve the above embodiments, such as Figure 11 As shown, this embodiment also provides a control method for a current control circuit with DC voltage sources connected in parallel. Based on the DC voltage sources with controlled charging and discharging power, a dual closed-loop control of voltage and current is adopted. The method includes:

[0103] S1. Based on different circuit connection methods, the allowable operating voltage range of the DC voltage source is determined according to formulas (2), (6), (9) or (12) based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply.

[0104] S2, obtain the operating voltage of the DC voltage source. If it exceeds the preset operating voltage range, disconnect the DC switch of the corresponding DC voltage source branch.

[0105] S3, performs closed-loop control on the output voltage of the medium-voltage DC bus to obtain the total current setpoint for all branches;

[0106] S4, allocate the current setpoint value for each branch according to the voltage level of the DC voltage source of each branch or the control command.

[0107] S5 performs closed-loop control on the inductor current of each branch according to the assigned current setpoint of each branch, and obtains the switching control signal of the power electronic half-bridge unit of each branch.

[0108] Specifically, for DC voltage sources with controlled charging and discharging power, such as battery clusters, a dual closed-loop control of voltage and current is adopted, and the control method steps are as follows:

[0109] For different circuit connection methods, based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply, the allowable operating voltage range of the DC voltage source is determined according to equations (2), (6), (9), or (12) respectively. min U max ];

[0110] Obtain the operating voltage of each DC voltage source. If the operating voltage exceeds the allowable range, disconnect the DC switch of the DC voltage source branch to disconnect the branch.

[0111] The output voltage of the medium-voltage DC bus is controlled in a closed loop to obtain the total current setpoint for all branches.

[0112] The current setpoint for each branch is assigned according to the voltage level of the DC voltage source of each branch, the SOC value, or the control command.

[0113] Typically, during discharge, a larger current setpoint is assigned to DC voltage source branches with higher voltage or higher SOC, and a smaller current setpoint is assigned to DC voltage source branches with lower voltage or lower SOC; during charging, a smaller current setpoint is assigned to DC voltage source branches with higher voltage or higher SOC, and a larger current setpoint is assigned to DC voltage source branches with lower voltage or lower SOC.

[0114] The inductor current of each branch is controlled in a closed loop according to the assigned current setpoint, thereby obtaining the switching control signal of the power electronic half-bridge unit of each branch.

[0115] Furthermore, for DC voltage sources such as photovoltaic strings that require maximum power point tracking (MPPT), the medium-voltage DC bus voltage is generally controlled by the downstream circuit, and the control method steps are as follows:

[0116] For different circuit connection methods, the allowable operating voltage range of the photovoltaic string is determined according to equations (2), (6), (9), or (12) based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply. min U max ];

[0117] The MPPT algorithm is used to obtain the inductor current setpoint for each photovoltaic string branch; since the output voltage of the photovoltaic string changes with the output current, it is necessary to ensure that the output voltage of the photovoltaic string remains within the allowable operating voltage range [U] during maximum power point tracking. min U max ] within.

[0118] The inductor current of each branch is controlled in a closed loop to ensure that it always operates at the current required to achieve the MPPT of the photovoltaic string, thereby obtaining the switching control signal of the power electronic half-bridge unit of each branch.

[0119] The control method of the current control circuit of the DC voltage source in parallel according to the embodiment of the present invention avoids the uneven current problem caused by the direct parallel connection of DC voltage sources such as photovoltaic or energy storage, while reducing the power and cost of DC / DC converter in string photovoltaic or energy storage system, and realizing effective control of the output current of each DC voltage source branch.

[0120] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0121] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

Claims

1. A current control circuit with DC voltage sources connected in parallel, characterized in that, The system includes a low-voltage DC power supply, a low-voltage DC bus capacitor, multiple power electronic half-bridge units, multiple branch inductors, and multiple DC switches. The output of the low-voltage DC power supply is connected in parallel to the low-voltage DC bus capacitor. The multiple power electronic half-bridge units are connected in parallel between the positive and negative terminals of the low-voltage DC bus. The neutral point output of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to form a current-sharing branch. The same terminal of the multiple DC voltage sources is connected to multiple current-sharing branches of the current control circuit. The other terminals of the multiple DC voltage sources are connected together to form one output terminal of the medium-voltage DC bus. At the same time, one terminal of the low-voltage DC bus is used as the other output terminal of the medium-voltage DC bus. A filter capacitor is connected in parallel between the positive and negative terminals of the medium-voltage DC bus to serve as the output terminal of the parallel DC voltage sources to supply power to the load. According to the preset connection method of the current control circuit, with the current flowing out of the half-bridge unit as the positive direction, the first... j The inductor voltage on this branch is: （1） When the half-bridge unit operates in PWM chopper mode, equation (1) satisfies the following conditions: S j = 1 hour u Lj > 0, the inductor current increases, in S j =0 u Lj < 0, the inductor current decreases, making the average inductor current controllable. The output voltage of the medium-voltage DC bus must meet the following requirements: （2） Based on the fact that the increase and decrease of the inductor current are equal during a switching cycle in steady state, according to equation (1): （3） in T on and T off The upper tube T of the half-bridge unit j The time for turning on and off within a switching cycle L j Given the branch inductance, obtain the steady-state switching function. S j The duty cycle in one switching cycle is: （4） According to the preset connection method of the current control circuit, at this time the first j The inductor voltage on this branch is: （5） The output voltage of the medium-voltage DC bus must meet the following requirements: （6） steady-state switching function S j The duty cycle in one switching cycle is: （7） According to the preset connection method of the current control circuit, at this time the first j The inductor voltage on this branch is: （8） The output voltage of the medium-voltage DC bus must meet the following requirements: （9） Steady-state real-time switching function S j The duty cycle in one switching cycle is: （10） According to the preset connection method of the current control circuit, at this time the first j The inductor voltage on this branch is: （11） The output voltage of the medium-voltage DC bus must meet the following requirements: （12） steady-state switching function S j The duty cycle in one switching cycle is: （13） 2. The current control circuit with parallel DC voltage sources according to claim 1, characterized in that, The positive terminals of multiple DC voltage sources are connected to the current sharing branch respectively, and the negative terminals of multiple DC voltage sources are connected together as the negative terminal of the medium-voltage DC bus. At the same time, the positive terminal of the low-voltage DC bus is used as the positive terminal of the medium-voltage DC bus, or the negative terminal of the low-voltage DC bus is used as the positive terminal of the medium-voltage DC bus.

3. The current control circuit with parallel DC voltage sources according to claim 1, characterized in that, Alternatively, the negative terminals of multiple DC voltage sources can be connected to the current sharing branch respectively, and the positive terminals of multiple DC voltage sources can be connected together as the positive terminal of the medium-voltage DC bus. At the same time, the negative terminal of the low-voltage DC bus can be used as the negative terminal of the medium-voltage DC bus, or the positive terminal of the low-voltage DC bus can be used as the negative terminal of the medium-voltage DC bus.

4. The current control circuit with parallel DC voltage sources according to claim 3, characterized in that, Assume the voltages of the multiple DC voltage sources are respectively U 1, U 2, ..., U n The highest voltage is U max The lowest voltage is U min The low-voltage DC bus voltage is U b The medium-voltage DC bus voltage is U d The power electronic half-bridge unit operates in a complementary state, and the upper transistor T of the power electronic half-bridge unit is defined. j The switching function is S j , j =1, 2, ..., n When the upper tube is on and the lower tube is off, S j =1, when the lower tube is on and the upper tube is off, S j =0.

5. The current control circuit with parallel DC voltage sources according to claim 1, characterized in that, When the DC voltage source is a photovoltaic string and requires a power supply with unidirectional output power, one of the two fully controlled switching devices in the power electronic half-bridge unit is replaced with a diode.

6. The current control circuit with parallel DC voltage sources according to claim 5, characterized in that, When the positive terminal of the DC voltage source is connected to the current sharing branch, the upper transistor of the power electronic half-bridge unit is replaced with a diode; when the negative terminal of the DC voltage source is connected to the current sharing branch, the lower transistor of the power electronic half-bridge unit is replaced with a diode.

7. The current control circuit with parallel DC voltage sources according to claim 1, characterized in that, The low-voltage DC power supply is an isolated AC / DC converter powered by AC power and an isolated DC / DC converter powered by DC power.

8. A control method for a current control circuit applied in parallel with the DC voltage source as described in claim 1, characterized in that, Based on the DC voltage source with controlled charging and discharging power, a dual closed-loop control of voltage and current is employed, the method comprising: Based on different circuit connection methods, the allowable operating voltage range of the DC voltage source is determined according to formulas (2), (6), (9) or (12) based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply. Obtain the operating voltage of the DC voltage source. If it exceeds the preset operating voltage range, disconnect the DC switch of the corresponding DC voltage source branch. The total current setpoint for all branches is obtained by performing closed-loop control on the output voltage of the medium-voltage DC bus. The current setpoint for each branch is assigned according to the voltage level of the DC voltage source of each branch or the control command. The inductor current of each branch is controlled in a closed loop according to the assigned current setpoint of each branch, thereby obtaining the switching control signal of the power electronic half-bridge unit of each branch.

9. The method according to claim 8, characterized in that, Based on the DC voltage source where the photovoltaic string has maximum power point tracking requirements, the medium-voltage DC bus voltage is controlled by the subsequent circuit. The method includes: Based on different circuit connection methods, the allowable operating voltage range of the photovoltaic string is determined according to formulas (2), (6), (9) or (12) based on the set medium-voltage DC bus voltage and the output voltage of the low-voltage DC power supply. The maximum power point tracking algorithm is used to obtain the inductor current setpoint for each photovoltaic string branch; Closed-loop control is performed on the inductor current of each photovoltaic string branch to obtain the switching control signal of the power electronic half-bridge unit of each photovoltaic string branch based on the control result.

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