Current control circuit for parallel direct-current voltage sources and control method

Abstract

Provided are a current control circuit for parallel direct-current voltage sources and a control method. The circuit comprises: an output end of a low-voltage direct-current power supply source being connected in parallel to a low-voltage direct-current bus capacitor; a plurality of power electronic half-bridge units being connected in parallel between the positive electrode and the negative electrode of a low-voltage direct-current bus; the neutral point output of each power electronic half-bridge unit being connected in series to a branch inductor and a direct-current switch so as to form a current balancing branch; same electrodes of a plurality of direct-current voltage sources being respectively connected to the plurality of current balancing branches of the current control circuit, and the other electrodes of the plurality of direct-current voltage sources being connected together to serve as an output end of a medium-voltage direct-current bus; and at the same time, one electrode of the low-voltage direct-current bus serving as another output end of the medium-voltage direct-current bus, and a filter capacitor being connected in parallel between the positive and negative electrodes of the medium-voltage direct-current bus to form an output end for the parallel connection of the direct-current voltage sources so as to supply power to a load.

Description

Current control circuit and control method for DC voltage sources in parallel

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application No. 202311255195X filed in China on September 26, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to the field of power electronics technology, and in particular to a current control circuit and a control method for parallel-connected DC voltage sources. Background Art

[0004] In the field of battery energy storage, to avoid parallel circulation issues between battery clusters, a string-type structure is often used. Each battery cluster is boosted by a full-power DC / DC converter before being connected in parallel. This effectively controls the charge and discharge currents and output voltage of each cluster. In the field of photovoltaic power generation, to achieve maximum power point tracking (MPPT) for photovoltaic panels, a common solution is to connect a full-power DC / DC converter to the output of each photovoltaic string. This DC / DC converter controls the output voltage and current of the photovoltaic string, thereby achieving maximum power point tracking. These scenarios essentially involve current control problems involving parallel DC voltage sources. While adding a full-power DC / DC converter can achieve better control performance, it also comes with disadvantages such as increased cost and reduced system efficiency. Improving the performance and efficiency of string-type energy storage and photovoltaic systems while reducing costs is a pressing challenge for the photovoltaic and energy storage industries. Figure 1 shows a schematic diagram of a string-type photovoltaic or energy storage system. Each photovoltaic or energy storage branch requires voltage stabilization via a full-power DC / DC converter before being connected in parallel. This DC / DC converter ensures output voltage stability while adapting to voltage variations across different photovoltaic or energy storage branches and controlling the output current of each. However, this DC / DC converter must operate simultaneously under high voltage and high current conditions, resulting in significant losses and high costs.

[0005] Summary of the Invention

[0006] The embodiments of the present disclosure aim to solve one of the technical problems in the related art at least to a certain extent.

[0007] In order to avoid the uneven current problem caused by the direct parallel connection of DC voltage sources such as photovoltaic or energy storage systems, while reducing the power and cost of DC / DC converters in string photovoltaic or energy storage systems, and achieving effective control of the output current of each DC voltage source branch, the embodiment of the present disclosure proposes a current control circuit and control method for parallel DC voltage sources. The idea is to first connect an inductor in series to each DC voltage source branch to turn it into 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 the purpose of precise current control of each DC voltage source.

[0008] Another object of the embodiments of the present disclosure is to provide a control method for a current control circuit in which a DC voltage source is connected in parallel.

[0009] To achieve the above-mentioned objectives, an embodiment of the present disclosure proposes, on the one hand, a current control circuit in which a DC voltage source is connected in parallel, comprising a low-voltage DC power supply, a low-voltage DC bus capacitor, a plurality of power electronic half-bridge units, a plurality of branch inductors, and a plurality of DC switches; the output end of the low-voltage DC power supply is connected in parallel to the low-voltage DC bus capacitor, and the plurality of power electronic half-bridge units are connected in parallel between the positive and negative poles 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 as a current sharing branch; the same pole of the plurality of DC voltage sources is respectively connected to the plurality of current sharing branches of the current control circuit, and the other poles of the plurality of DC voltage sources are connected together as an output end of the medium-voltage DC bus, while one pole of the low-voltage DC bus is used as another output end of the medium-voltage DC bus, and a filter capacitor is connected in parallel between the positive and negative poles of the medium-voltage DC bus as the output end of the parallel-connected DC voltage sources to supply power to the load.

[0010] The current control circuit of the embodiment of the present disclosure in parallel with the DC voltage source may also have the following additional technical features:

[0011] In one embodiment of the present disclosure, the positive poles of the multiple DC voltage sources are respectively connected to the current sharing branch, and the negative poles of the multiple DC voltage sources are connected together as the negative pole of the medium-voltage DC bus. At the same time, the positive pole of the low-voltage DC bus is used as the positive pole of the medium-voltage DC bus, or the negative pole of the low-voltage DC bus is used as the positive pole of the medium-voltage DC bus.

[0012] In one embodiment of the present disclosure, alternatively, the negative poles of the multiple DC voltage sources are respectively connected to the current sharing branch, and the positive poles of the multiple DC voltage sources are connected together as the positive pole of the medium-voltage DC bus, and at the same time, the negative pole of the low-voltage DC bus is used as the negative pole of the medium-voltage DC bus, or the positive pole of the low-voltage DC bus is used as the negative pole of the medium-voltage DC bus.

[0013] In one embodiment of the present disclosure, it is assumed that the voltages of the plurality of DC voltage sources are U1, U2, ..., U n , where the highest voltage is U max , the minimum 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 works in a complementary state, and the upper tube T of the power electronic half-bridge unit is defined as j The switching function is S j , j=1,2,……,n, when the upper tube is turned on and the lower tube is turned off, S j =1, when the lower tube is turned on and the upper tube is turned off, S j =0.

[0014] In one embodiment of the present disclosure, according to the first circuit connection mode of the current control circuit, the current flowing out of the half-bridge unit is considered as the positive direction. At this time, the inductor voltage on the j-th branch is: u Lj =U d -U j -(1-S j )U b (1);

[0015] Based on the fact that the half-bridge unit works in PWM chopping mode, equation (1) satisfies the following conditions: j =1 when u Lj >0, the inductor current increases, and S j =0 when u Lj <0, the inductor current decreases, making the average value of the inductor current controllable, and the medium voltage DC bus output voltage must meet the following requirements: U max d b +U min (2);

[0016] Based on the fact that the increase and decrease of the steady-state inductor current in one switching cycle are equal, according to formula (1), we can get:

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

[0018] According to the second circuit connection mode of the current control circuit, the inductor voltage on the j-th branch is: u Lj =U d -U j +S j ·U b (5);

[0019] The medium voltage DC bus output voltage must meet the following requirements: U max -U b d min (6);

[0020] Based on the steady-state switching function S j The duty cycle in one switching cycle is: j =(U j -U d ) / U b (7);

[0021] According to the third circuit connection mode of the above current control circuit, the inductor voltage on the j-th branch is: u Lj =U j +S j ·U b -U d (8);

[0022] The medium voltage DC bus output voltage must meet the following requirements: U max d b +U min (9);

[0023] Based on the steady-state switching function S j The duty cycle in one switching cycle is: j =(U d -U j ) / U b (10);

[0024] According to the fourth circuit connection mode of the above current control circuit, the inductor voltage on the j-th branch is: u Lj =U j -(1-S​​​​j )·U b -U d (11);

[0025] The medium voltage DC bus output voltage must meet the following requirements: U max -U b d min (12);

[0026] Based on the steady-state switching function S j The duty cycle in one switching cycle is: j =(U d +U b -U j ) / U b (13).

[0027] In one embodiment of the present disclosure, based on the fact that the DC voltage source is a photovoltaic string and a power source with unidirectional output power is required, one of the two fully controlled switching devices of the power electronic half-bridge unit is replaced with a diode.

[0028] In one embodiment of the present disclosure, based on the positive electrode of the DC voltage source being connected to the above-mentioned current sharing branch, the upper tube of the power electronic half-bridge unit is replaced with a diode; based on the negative electrode of the DC voltage source being connected to the above-mentioned current sharing branch, the lower tube of the power electronic half-bridge unit is simplified to a diode.

[0029] In one embodiment of the present disclosure, the low-voltage DC power supply is an isolated AC / DC converter powered by an AC power supply or an isolated DC / DC converter powered by a DC power supply.

[0030] To achieve the above objectives, another embodiment of the present disclosure provides a method for controlling a current control circuit in parallel with a DC voltage source. For the DC voltage source with controlled charge and discharge power, dual closed-loop voltage and current control is adopted. The method includes:

[0031] Based on different circuit connection modes, according to 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 formula (2), (6), (9) or (12);

[0032] Obtaining an operating voltage of a DC voltage source, and disconnecting a DC switch of a corresponding DC voltage source branch if the operating voltage exceeds a preset operating voltage range;

[0033] Performing closed-loop control on the output voltage of the medium voltage DC bus to obtain a given total current value of all branches;

[0034] ​​Allocating a current set value to each branch according to the voltage level of the DC voltage source of each branch or a control instruction;

[0035] The inductor current of each branch is closed-loop controlled according to the assigned current set value of each branch, so as to obtain a switch control signal of the power electronic half-bridge unit of each branch.

[0036] In one embodiment of the present disclosure, based on the DC voltage source having a maximum power point tracking requirement for a photovoltaic string, the medium voltage DC bus voltage is controlled by a subsequent circuit, and the method includes:

[0037] Based on the different circuit connection modes, according to 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 photovoltaic string is determined according to formula (2), (6), (9) or (12);

[0038] A maximum power point tracking algorithm is used to obtain a given value of the inductor current of each photovoltaic string branch;

[0039] The inductive current of each photovoltaic string branch is closed-loop controlled to obtain a switch control signal of the power electronic half-bridge unit of each photovoltaic string branch according to the control result.

[0040] The current control circuit and control method for parallel DC voltage sources in the embodiments of the present disclosure can implement a current control circuit and control for non-direct parallel DC voltage sources such as battery clusters and photovoltaic strings. 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 converted into parallel connection of current sources. The current control is simple and highly accurate. Compared with the traditional string structure, the cost and loss of the DC / DC converter can also be greatly reduced.

[0041] Additional aspects and advantages of the present disclosure will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The above and / or additional aspects and advantages of the present disclosure will become apparent and readily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0043] FIG1 is a related art group string voltage source parallel circuit;

[0044] FIG2 is a structural diagram of a DC voltage source parallel current control circuit according to an embodiment of the present disclosure;

[0045] FIG3 is a schematic diagram of a first connection mode of a DC voltage source parallel current control circuit according to an embodiment of the present disclosure;

[0046] FIG4 is a schematic diagram of a second connection mode of a DC voltage source parallel current control circuit according to an embodiment of the present disclosure;

[0047] FIG5 is a schematic diagram of a third connection mode of a DC voltage source parallel current control circuit according to an embodiment of the present disclosure;

[0048] FIG6 is a schematic diagram of a fourth connection mode of a DC voltage source parallel current control circuit according to an embodiment of the present disclosure;

[0049] FIG7 is a schematic diagram of a first connection mode of a DC voltage source parallel current control circuit for a photovoltaic string according to an embodiment of the present disclosure;

[0050] FIG8 is a schematic diagram of a second connection mode of a DC voltage source parallel current control circuit for a photovoltaic string according to an embodiment of the present disclosure;

[0051] FIG9 is a schematic diagram of a third connection method of a DC voltage source parallel current control circuit for a photovoltaic string according to an embodiment of the present disclosure;

[0052] FIG10 is a schematic diagram of a fourth connection mode of a DC voltage source parallel current control circuit for a photovoltaic string according to an embodiment of the present disclosure;

[0053] FIG11 is a flow chart of a method for controlling a current control circuit in parallel with a DC voltage source according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0054] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in the present disclosure may be combined with each other. The present disclosure will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0055] In order to enable those skilled in the art to better understand the solutions of the present disclosure, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the embodiments described are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present disclosure.

[0056] The current control circuit and control method for parallel connection of DC voltage sources according to embodiments of the present disclosure will be described below with reference to the accompanying drawings.

[0057] FIG2 is a flow chart of a current control circuit of a DC voltage source in parallel with an embodiment of the present disclosure. As shown in FIG2 , it includes a low-voltage DC power supply, a low-voltage DC bus capacitor C b , n power electronic half-bridge units (n is a positive integer), n branch inductors, and n DC switches. The output end of the low-voltage DC power supply is connected in parallel with the low-voltage DC bus capacitor Cb To provide a stable DC voltage, all power electronic half-bridge units are connected in parallel between the positive LVDC+ and negative LVDC- poles 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 n parallel DC voltage sources, the same pole of the n DC voltage sources is connected to the n current-sharing branches of the current control circuit. The other poles of the n DC voltage sources are connected together to form one output terminal of the medium-voltage DC bus. At the same time, one pole 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 poles of the medium-voltage DC bus, which serves as the output terminal of the parallel DC voltage sources to power the load.

[0058] In some embodiments, the positive poles of n DC voltage sources can be connected to the current sharing branches respectively, as shown in Figures 3 and 4. At this time, the negative poles of the n DC voltage sources are connected together as the negative pole DC- of the medium voltage DC bus, and the positive pole of the low voltage DC bus is used as the positive pole DC+ of the medium voltage DC bus, as shown in Figure 3, or the negative pole of the low voltage DC bus is used as the positive pole DC+ of the medium voltage DC bus, as shown in Figure 4.

[0059] In some embodiments, the negative poles of n DC voltage sources can also be connected to the current sharing branches respectively, as shown in Figures 5 and 6. At this time, the positive poles of the n DC voltage sources are connected together as the positive pole DC+ of the medium voltage DC bus, and the negative pole of the low voltage DC bus is used as the negative pole DC- of the medium voltage DC bus, as shown in Figure 5, or the positive pole of the low voltage DC bus is used as the negative pole DC- of the medium voltage DC bus, as shown in Figure 6.

[0060] In one embodiment of the present disclosure, it is assumed that the voltages of the multiple DC voltage sources are U1, U2, ..., U n , where the highest voltage is U max , the minimum 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 works in a complementary state, defining the power electronic half-bridge unit upper tube T j The switching function is S j , j=1,2,……,n, when the upper tube is turned on and the lower tube is turned off, S j =1, when the lower tube is turned on and the upper tube is turned off, S j =0.

[0061] In one embodiment of the present disclosure, taking the connection mode shown in FIG3 as an example, the current flowing out of the half-bridge unit is considered as the positive direction. At this time, the inductor voltage on the j-th branch can be written as: u Lj =U d -U j-(1-S j )U b (1).

[0062] In order to make the inductor current controllable, when the half-bridge unit works in PWM chopping mode, equation (1) must satisfy the following conditions: j =1 when u Lj >0, the inductor current increases, and S j =0 when u Lj <0, the inductor current decreases, so that the average value of the inductor current can be controlled. Therefore, the medium voltage DC bus output voltage must meet the following requirements: U max d b +U min (2).

[0063] From formula (2), it can be concluded that the medium voltage DC bus output voltage U 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 maximum DC power supply voltage U max With the minimum voltage U min difference.

[0064] Since the increase and decrease of the inductor current in a switching cycle are equal in steady state, it can be deduced from formula (1):

[0065] Where T on and T off They are the upper tube T of the half-bridge unit j The time of turning on and off in a switching cycle, L j is the branch inductance, so the switching function S in steady state can be derived j The duty cycle in one switching cycle is:

[0066] Substituting equation (2) into equation (4), we can get the duty cycle d j The range is between 0 and 1, which proves the correctness of the analysis results.

[0067] In one embodiment of the present disclosure, for the connection mode shown in FIG4 , the inductor voltage on the j-th branch can be written as: u Lj =U d -U j +S j ·U b (5).

[0068] The same idea can be used to deduce that the medium voltage DC bus output voltage must meet the following requirements: U​​max -U b d min (6).

[0069] Switching function S in steady state j The duty cycle in one switching cycle is: j =(U j -U d ) / U b (7).

[0070] Substituting equation (6) into equation (7), we can get the duty cycle d j The range is between 0 and 1, which proves the correctness of the analysis results.

[0071] In one embodiment of the present disclosure, for the connection mode shown in FIG5 , the inductor voltage on the j-th branch can be written as: u Lj =U j +S j ·U b -U d (8).

[0072] The same idea can be used to deduce that the medium voltage DC bus output voltage must meet the following requirements: U max d b +U min (9).

[0073] At this time, the switching function S j The duty cycle in one switching cycle is: j =(U d -U j ) / U b (10).

[0074] Substituting equation (9) into equation (10), we can get the duty cycle d j The range is between 0 and 1, which proves the correctness of the analysis results.

[0075] In one embodiment of the present disclosure, for the connection mode shown in FIG6 , the inductor voltage on the j-th branch can be written as: u Lj =U j -(1-S j )·U b -U d (11).

[0076] The same idea can be used to deduce that the medium voltage DC bus output voltage must meet the following requirements: U max ​​​​-U b d min (12).

[0077] Switching function S in steady state j The duty cycle in one switching cycle is: j =(U d +U b -U j ) / U b (13).

[0078] Substituting equation (12) into equation (13), we can get the duty cycle d j The range is between 0 and 1, which proves the correctness of the analysis results.

[0079] Therefore, for the four different connection methods mentioned above, 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). If the DC voltage source voltage of a branch is too high or too low to satisfy equations (2), (6), (9), or (12), the DC switch of the branch can be disconnected to cut off the branch.

[0080] In some embodiments, when the DC power source is a power source such as a photovoltaic string that only requires unidirectional output power, 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 electrode 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, as shown in Figures 7 and 8, the upper transistor of the power electronic half-bridge unit can be simplified to a diode. When the negative electrode of the DC voltage source is connected to the current sharing branch, since the current always flows from the current sharing branch into the photovoltaic string, as shown in Figures 9 and 10, the lower transistor of the power electronic half-bridge unit can be simplified to a diode.

[0081] In some embodiments, the low-voltage DC power supply is an isolated AC / DC converter powered by an AC power supply, or an isolated DC / DC converter powered by a DC power supply.

[0082] ​​According to the current control circuit of the DC voltage source in parallel of the embodiment of the present disclosure, by connecting an inductor and a power electronic half-bridge unit in series to each DC voltage source branch, the DC voltage source in parallel is converted into a current source in parallel, and the current closed-loop control has high precision and good current control performance; the proposed current control circuit has low voltage and low power, avoiding the full-power DC / DC converter in the traditional string-type solution, greatly reducing the system volume and cost, and improving efficiency; the proposed current control circuit adopts a common DC bus structure, and all branches only need a common independent DC power supply, which is low in cost.

[0083] In order to implement the above embodiment, as shown in Figure 11, this embodiment also provides a control method for a current control circuit in parallel with a DC voltage source. Based on the DC voltage source with controlled charging and discharging power, a voltage and current dual closed-loop control is adopted, and the method includes steps S1 to S5.

[0084] S1, based on different circuit connection modes, according to the set medium voltage DC bus voltage and the output voltage of the low voltage DC power supply, determines the allowable operating voltage range of the DC voltage source according to formula (2), (6), (9) or (12).

[0085] S2, obtaining the operating voltage of the DC voltage source, and if it exceeds a preset operating voltage range, disconnecting the DC switch of the corresponding DC voltage source branch.

[0086] S3, performs closed-loop control on the output voltage of the medium voltage DC bus to obtain a given total current value of all branches.

[0087] S4, allocating a current set value to each branch according to the voltage level of the DC voltage source of each branch or the control instruction.

[0088] S5 , performing closed-loop control on the inductor current of each branch according to the assigned current given value of each branch, and obtaining a switch control signal of the power electronic half-bridge unit of each branch.

[0089] In some specific embodiments, for a DC voltage source with controlled charge and discharge power, such as a battery cluster, a voltage-current dual closed-loop control is adopted, and the control method steps are as follows:

[0090] For different circuit connection modes, according to 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 [U min , U max ].

[0091] The operating voltage of each DC voltage source is obtained. If the operating voltage exceeds the allowable operating voltage range, the DC switch of the DC voltage source branch is disconnected to cut off the branch.

[0092] The output voltage of the medium voltage DC bus is closed-loop controlled to obtain the total current set value of all branches.

[0093] The current set value of each branch is allocated according to the voltage level of the DC voltage source of each branch or the SOC size or control instruction.

[0094] In some embodiments, under discharging conditions, a larger current set value is assigned to a DC voltage source branch with a higher voltage or a higher SOC, and a smaller current set value is assigned to a DC voltage source branch with a lower voltage or a lower SOC; under charging conditions, a smaller current set value is assigned to a DC voltage source branch with a higher voltage or a higher SOC, and a larger current set value is assigned to a DC voltage source branch with a lower voltage or a lower SOC.

[0095] The inductor current of each branch is closed-loop controlled according to the assigned current given value, thereby obtaining a switch control signal of the power electronic half-bridge unit of each branch.

[0096] In some embodiments, for a DC voltage source with maximum power point tracking (MPPT) requirements, such as a photovoltaic string, the medium voltage DC bus voltage is generally controlled by a subsequent circuit, and the control method steps are as follows:

[0097] For different circuit connection modes, according to 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 PV string [U min , U max ].

[0098] The MPPT algorithm is used to obtain the given value of the inductor current of 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 is within the allowable operating voltage range [U min , U max ] within.

[0099] The inductive current of each branch is closed-loop controlled so that it always operates at the current required to achieve MPPT of the photovoltaic string, thereby obtaining the switching control signal of the power electronic half-bridge unit of each branch.

[0100] According to the control method of the current control circuit of the DC voltage source in parallel according to the embodiment of the present disclosure, the uneven current problem caused by the direct parallel connection of DC voltage sources such as photovoltaic or energy storage is avoided, while the power and cost of the DC / DC converter in the string photovoltaic or energy storage system are reduced, and effective control of the output current of each DC voltage source branch is achieved.

[0101] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and features of different embodiments or examples, unless they are mutually inconsistent.

[0102] 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 being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. Throughout the present disclosure, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

Claims

1. A current control circuit with a DC voltage source connected in parallel, characterized in that: The invention comprises a low-voltage DC power supply, a low-voltage DC bus capacitor, a plurality of power electronic half-bridge units, a plurality of branch inductors and a plurality of DC switches; the output end of the low-voltage DC power supply is connected in parallel with the low-voltage DC bus capacitor, a plurality of power electronic half-bridge units are connected in parallel between the positive and negative poles of the low-voltage DC bus, and the neutral point output of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch as a current sharing branch; the same pole of the plurality of DC voltage sources is respectively connected to the plurality of current sharing branches of the current control circuit, the other poles of the plurality of DC voltage sources are connected together as an output end of a medium-voltage DC bus, and one pole of the low-voltage DC bus is used as another output end of the medium-voltage DC bus, and a filter capacitor is connected in parallel between the positive and negative poles of the medium-voltage DC bus as the output end of the DC voltage sources connected in parallel to supply power to the load.

2. The current control circuit of the DC voltage source in parallel according to claim 1, characterized in that: The positive poles of the multiple DC voltage sources are respectively connected to the current sharing branches, and the negative poles of the multiple DC voltage sources are connected together as the negative pole of the medium voltage DC bus. At the same time, the positive pole of the low voltage DC bus is used as the positive pole of the medium voltage DC bus, or the negative pole of the low voltage DC bus is used as the positive pole of the medium voltage DC bus.

3. The current control circuit of the DC voltage source in parallel according to claim 1, characterized in that: Alternatively, the negative poles of the multiple DC voltage sources are respectively connected to the current sharing branches, and the positive poles of the multiple DC voltage sources are connected together as the positive pole of the medium voltage DC bus, and the negative pole of the low voltage DC bus is used as the negative pole of the medium voltage DC bus, or the positive pole of the low voltage DC bus is used as the negative pole of the medium voltage DC bus.

4. The current control circuit in which a DC voltage source is connected in parallel according to any one of claims 1 to 3, characterized in that: Assume that the voltages of the plurality of DC voltage sources are U1, U2, ..., U n , where the highest voltage is U max , the minimum 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 works in a complementary state, and the upper tube T of the power electronic half-bridge unit is defined as j The switching function is S j , j = 1, 2, ..., n, when the upper tube is turned on and the lower tube is turned off, S j =1, when the lower tube is turned on and the upper tube is turned off, S j =0.

5. The current control circuit of the DC voltage source in parallel according to claim 4, characterized in that: According to the first circuit connection mode of the current control circuit, the current flowing out of the half-bridge unit is regarded as the positive direction. At this time, the inductor voltage on the j-th branch is: in Lj =U d -IN j -(1-S j )IN b (1); Based on the fact that the half-bridge unit operates in PWM chopping mode, equation (1) satisfies S j =1 when u Lj >0, the inductor current increases, at S j = 0 when u Lj <0, the inductor current decreases, making the average value of the inductor current controllable, and the medium voltage DC bus output voltage needs to meet: IN max d b +U min (2);​​ Based on the fact that the increase and decrease of the steady-state inductor current in one switching cycle are equal, according to formula (1), we can get: Where T on and T off They are the upper tubes T of the half-bridge unit. j The time of on and off in a switching cycle, L j is the branch inductance, based on the steady-state switching function S j The duty cycle in one switching cycle is: According to the second circuit connection mode of the current control circuit, the inductor voltage on the j-th branch is: in Lj =U d -IN j +S j ·IN b (5); The medium voltage DC bus output voltage must meet the following requirements: IN max -IN b d min (6);​​ Based on the steady-state switching function S j The duty cycle in one switching cycle is: d j =(U j -U d ) / U b (7); According to the third circuit connection mode of the current control circuit, the inductor voltage on the j-th branch is: in Lj =U j +S j ·IN b -IN d (8); The medium voltage DC bus output voltage must meet the following requirements: IN max d b +U min (9);​​ Based on the steady-state switching function S j The duty cycle in one switching cycle is: d j =(U d -U j ) / U b (10); According to the fourth circuit connection mode of the current control circuit, the inductor voltage on the j-th branch is: in Lj =U j -(1-S j )·IN b -IN d (11); The medium voltage DC bus output voltage must meet the following requirements: IN max -IN b d min (12);​​ Based on the steady-state switching function S j The duty cycle in one switching cycle is: d j =(U d +U b -U j ) / U b (13) 6. The current control circuit of the DC voltage source in parallel according to claim 1, characterized in that: Based on the fact that the DC voltage source is a photovoltaic string and a power source with unidirectional output power is required, one of the two fully controlled switching devices of the power electronic half-bridge unit is replaced by a diode.

7. The current control circuit of the DC voltage source in parallel according to claim 6, characterized in that: Based on the positive electrode of the DC voltage source being connected to the current sharing branch, the upper tube of the power electronic half-bridge unit is replaced by a diode; based on the negative electrode of the DC voltage source being connected to the current sharing branch, the lower tube of the power electronic half-bridge unit is replaced by a diode.

8. The current control circuit in which a DC voltage source is connected in parallel according to any one of claims 1 to 7, characterized in that: The low-voltage DC power supply is an isolated AC / DC converter powered by an AC power supply or an isolated DC / DC converter powered by a DC power supply.

9. A control method for a current control circuit in parallel with a DC voltage source as claimed in any one of claims 1 to 8, characterized in that: Based on the DC voltage source with controlled charge and discharge power, voltage and current dual closed-loop control is adopted, and the method includes: Based on different circuit connection modes, according to 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 formula (2), (6), (9) or (12); Obtaining the operating voltage of the DC voltage source, and disconnecting the DC switch of the corresponding DC voltage source branch when the operating voltage exceeds a preset operating voltage range; Performing closed-loop control on the output voltage of the medium voltage DC bus to obtain a given total current value of all branches; Allocating a current given value to each branch according to the voltage level of the DC voltage source of each branch or a control instruction; The inductor current of each branch is closed-loop controlled according to the assigned current set value of each branch, so as to obtain a switch control signal of the power electronic half-bridge unit of each branch.

10. The method according to claim 9, characterized in that Based on the DC voltage source having the maximum power point tracking requirement of the photovoltaic string, the medium voltage DC bus voltage is controlled by the subsequent circuit, and the method includes: Based on the different circuit connection modes, according to 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 photovoltaic string is determined according to formula (2), (6), (9) or (12); A maximum power point tracking algorithm is used to obtain a given value of the inductor current of each photovoltaic string branch; The inductive current of each photovoltaic string branch is closed-loop controlled to obtain a switch control signal of the power electronic half-bridge unit of each photovoltaic string branch according to the control result.

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