A photovoltaic system and a midpoint balancing method

Abstract

The present application discloses a photovoltaic system and a midpoint balancing method. The system includes: a first capacitor, a second capacitor, an energy storage element, and a controller; both ends of the first capacitor are respectively connected to BUS+ and the output midpoint of the first DCDC circuit, both ends of the second capacitor are respectively connected to the output midpoint of the first DCDC circuit and the N line; the energy storage element is connected to the output midpoint of the first DCDC circuit; the controller is configured to transfer the energy of the first capacitor to the energy storage element when the voltage of the first capacitor is greater than the voltage of the second capacitor, and then transfer the energy of the energy storage element to the second capacitor; when the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor. The present application adds an energy storage element to realize the transfer of energy on the capacitors, balance the voltage at the output midpoint of the first DCDC circuit, and protect the switching devices.

Description

Technical Field

[0001] This application relates to the technical field of photovoltaic power generation, and particularly to a photovoltaic system and a midpoint balancing method. Background Art

[0002] Currently, in order to reduce the voltage stress of each switching device in a power converter in a photovoltaic system, bipolar photovoltaic systems have emerged. For example, for a ±1500V photovoltaic system, the bus voltage is 3000V, and the system voltage is designed according to half of the bus voltage. Since the system voltage is designed according to half of the bus voltage, in terms of safety regulations requirements, it is necessary to ensure that the system voltage does not exceed half of the bus voltage (if taking ±1500V as an example, the system voltage does not exceed 1500V).

[0003] Currently, a bipolar photovoltaic system includes a first DCDC circuit and a second DCDC circuit. The positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of a photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-. Since the bus voltage of the photovoltaic system is relatively high, taking the first DCDC circuit generally selecting a three-level Boost circuit as an example, under normal circumstances, the voltage between the positive output terminal and the midpoint of the output terminal of the first DCDC circuit is equal to the voltage between the negative output terminal and the midpoint of the output terminal, that is, the potential of the midpoint of the output terminal of the first DCDC circuit is balanced.

[0004] However, this circuit has the following problems in actual application. In some working conditions, the clamping diode will conduct, that is, the potential of the midpoint of the output terminal of the first DCDC circuit is unbalanced, forming a current path from the midpoint of the output terminal of the first DCDC circuit to the N line, resulting in excessive voltage stress on the switching device in parallel with the half bus, leading to device damage. Summary of the Invention

[0005] This application provides a photovoltaic system and a midpoint balancing method, which can adjust the potential of the midpoint of the output terminal of the first DCDC circuit to achieve balance when the potential of the midpoint of the output terminal of the first DCDC circuit is unbalanced, and protect the switching device.

[0006] This application provides a photovoltaic system, including: a first DCDC circuit, a second DCDC circuit, a first capacitor, a second capacitor, an energy storage element, and a controller;

[0007] The positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of a photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+;

[0008] The positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-.

[0009] Both ends of the first capacitor are respectively connected to the BUS+ and the midpoint of the output terminal of the first DCDC circuit, and both ends of the second capacitor are respectively connected to the midpoint of the output terminal of the first DCDC circuit and the negative output terminal of the first DCDC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit.

[0010] The controller is configured to transfer the energy of the first capacitor to the energy storage element when the voltage of the first capacitor is greater than the voltage of the second capacitor, and then transfer the energy of the energy storage element to the second capacitor; when the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor.

[0011] Preferably, it further includes: a first switch unit and a second switch unit;

[0012] The first end of the first switch unit is connected to the BUS+, and the second end of the first switch unit is connected to the midpoint of the output terminal of the first DCDC circuit;

[0013] The first end of the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit, and the second end of the energy storage element is connected to the second end of the first switch unit;

[0014] The first end of the second switch unit is connected to the second end of the first switch unit, and the second end of the second switch unit is connected to the N line;

[0015] The controller is specifically configured to, when the voltage of the first capacitor is greater than the voltage of the second capacitor, transfer the energy of the first capacitor to the energy storage element by controlling the first switch unit to close; when the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element by controlling the second switch unit to close.

[0016] Preferably, the first switch unit includes a first switch tube, and the second switch unit includes a second switch tube;

[0017] Both the first switch tube and the second switch tube include anti-parallel diodes;

[0018] The controller is specifically configured to control both the first switching transistor and the second switching transistor to be turned off when transferring the energy of the energy storage element to the second capacitor; and to control both the first switching transistor and the second switching transistor to be turned off when transferring the energy of the energy storage element to the first capacitor.

[0019] Preferably, the first switching unit includes a first switching transistor and a second switching transistor connected in series; the second switching transistor includes a third switching transistor and a fourth switching transistor connected in series; both the first switching transistor and the second switching transistor include anti-parallel diodes; both the third switching transistor and the fourth switching transistor include anti-parallel diodes;

[0020] The controller is specifically configured to control the first switching transistor, the second switching transistor, the third switching transistor, and the fourth switching transistor to be all turned off when transferring the energy of the energy storage element to the second capacitor; and to control the first switching transistor, the second switching transistor, the third switching transistor, and the fourth switching transistor to be all turned off when transferring the energy of the energy storage element to the first capacitor.

[0021] Preferably, it further includes: a first diode and a second diode;

[0022] The cathode of the first diode is connected to the common terminal of the first switching transistor and the second switching transistor, and the anode of the first diode is connected to the midpoint of the output terminal of the first DCDC circuit;

[0023] The anode of the second diode is connected to the common terminal of the third switching transistor and the fourth switching transistor, and the anode of the second diode is connected to the midpoint of the output terminal of the first DCDC circuit.

[0024] Preferably, the midpoint of the input terminal of the second DCDC circuit is connected to the anode of the first diode.

[0025] Preferably, the energy storage element includes an inductor.

[0026] Preferably, when the voltage of the first capacitor is greater than the voltage of the second capacitor, the first switching unit includes a first switching transistor, and the second switching unit includes a second diode;

[0027] The controller is specifically configured to control the first switching transistor to be turned on when transferring the energy of the first capacitor to the energy storage element.

[0028] Preferably, when the voltage of the second capacitor is greater than the voltage of the first capacitor, the first switching unit includes a first diode, and the second switching unit includes a second switching transistor;

[0029] The controller is specifically configured to control the second switch tube to close when transferring the energy of the second capacitor to the energy storage element.

[0030] Preferably, it further includes: a first DCAC circuit and a second DCAC circuit;

[0031] The positive input terminal of the first DCAC circuit is connected to the BUS+, and the negative input terminal of the first DCAC circuit is connected to the N line;

[0032] The positive input terminal of the second DCAC circuit is connected to the N line, and the negative input terminal of the second DCAC circuit is connected to the BUS-.

[0033] The present application provides a midpoint balancing method for a photovoltaic system. The photovoltaic system includes: a first DCDC circuit, a second DCDC circuit, a first capacitor, a second capacitor, and an energy storage element; the positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-; both ends of the first capacitor are respectively connected to the BUS+ and the midpoint of the output terminal of the first DCDC circuit, both ends of the second capacitor are respectively connected to the midpoint of the output terminal of the first DCDC circuit and the negative output terminal of the first DCDC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit;

[0034] The method includes:

[0035] When the voltage of the first capacitor is greater than the voltage of the second capacitor, transfer the energy of the first capacitor to the energy storage element, and then transfer the energy of the energy storage element to the second capacitor;

[0036] When the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor.

[0037] Preferably, the photovoltaic system further includes: a first switch unit and a second switch unit; the first end of the first switch unit is connected to the BUS+, and the second end of the first switch unit is connected to the midpoint of the output terminal of the first DCDC circuit; the first end of the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit, and the second end of the energy storage element is connected to the second end of the first switch unit; the first end of the second switch unit is connected to the second end of the first switch unit, and the second end of the second switch unit is connected to the N line;

[0038] When the voltage of the first capacitor is greater than the voltage of the second capacitor, transferring the energy of the first capacitor to the energy storage element specifically includes:

[0039] When the voltage of the first capacitor is greater than the voltage of the second capacitor, by controlling the first switch unit to close, transferring the energy of the first capacitor to the energy storage element;

[0040] When the voltage of the second capacitor is greater than the voltage of the first capacitor, transferring the energy of the second capacitor to the energy storage element specifically includes:

[0041] When the voltage of the second capacitor is greater than the voltage of the first capacitor, by controlling the second switch unit to close, transferring the energy of the second capacitor to the energy storage element.

[0042] It can be seen from the above technical solutions that the present application has at least the following advantages:

[0043] For the photovoltaic system provided by the present application, when the voltage on the first capacitor is greater than the voltage on the second capacitor, it indicates that there is an imbalance in the voltage at the midpoint of the output terminal of the first DCDC circuit. Therefore, in order to balance the voltage at the midpoint of the output terminal of the first DCDC circuit, it is necessary to reduce the voltage on the first capacitor and increase the voltage on the second capacitor. Therefore, the present application adds an energy storage element. In order not to cause energy loss, the excess energy on the first capacitor is transferred to the energy storage element and then transferred from the energy storage element to the second capacitor until the voltage on the first capacitor is equal to the voltage on the second capacitor. Similarly, when the voltage on the second capacitor is greater than the voltage on the first capacitor, it is necessary to reduce the voltage on the second capacitor and increase the voltage on the first capacitor. Therefore, the excess energy on the second capacitor is transferred to the energy storage element and then transferred from the energy storage element to the first capacitor until the voltage of the first capacitor is equal to the voltage of the second capacitor, finally balancing the voltage at the midpoint of the output terminal of the first DCDC circuit, equalizing the voltage stress of the switching device connected in parallel with the half-bus, and protecting the switching device. Description of the Drawings

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0045] Figure 1 Schematic diagram of a photovoltaic system provided by an embodiment of the present application;

[0046] Figure 2 For Figure 1 The current path diagram when the corresponding clamping diode is conducting;

[0047] Figure 3 The schematic diagram of another photovoltaic system provided by the embodiment of the present application;

[0048] Figure 4 The schematic diagram of yet another photovoltaic system provided by the embodiment of the present application;

[0049] Figure 5A The schematic diagram of yet another photovoltaic system provided by the embodiment of the present application;

[0050] Figure 5B The schematic diagram of another photovoltaic system provided by the embodiment of the present application;

[0051] Figure 5C The schematic diagram of yet another photovoltaic system provided by the embodiment of the present application;

[0052] Figure 6 The schematic diagram of yet another photovoltaic system provided by the embodiment of the present application;

[0053] Figure 7 The schematic diagram of another photovoltaic system provided by the embodiment of the present application;

[0054] Figure 8 The flowchart of a midpoint balancing method for a photovoltaic system provided by the embodiment of the present application. Detailed implementation manners

[0055] In order to enable those skilled in the art to well understand the technical solutions provided by the present application, the application scenarios of the photovoltaic system provided by the present application will be introduced first below.

[0056] Refer to Figure 1 , which is the schematic diagram of a photovoltaic system provided by the present application.

[0057] The photovoltaic system provided by the embodiment of the present application includes a positive bus BUS+, an N line, and a BUS-. The voltage between BUS+ and the N line is equal to the voltage between BUS- and the N line under normal circumstances. For example, the voltage of BUS+ is 1500V, the voltage of BUS- is -1500V, and the voltage of the N line is 0V.

[0058] The photovoltaic system provided by the present application includes: a first DCDC circuit 10, a second DCDC circuit 20, a first DCAC circuit 30, and a second DCAC circuit 40;

[0059] The positive input terminal and the negative input terminal of the first DCDC circuit 10 are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit 10 is connected to the positive bus BUS+;

[0060] The positive input terminal and the negative input terminal of the second DCDC circuit 20 are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit 10; the positive output terminal of the second DCDC circuit 20 is connected to the N line, and the positive output terminal of the second DCDC circuit 20 is connected to the negative bus BUS-.

[0061] The positive input terminal of the first DCAC circuit 30 is connected to BUS+, and the negative input terminal of the first DCAC circuit 30 is connected to the N line;

[0062] The positive input terminal of the second DCAC circuit 40 is connected to the N line, and the negative input terminal of the second DCAC circuit 40 is connected to BUS-.

[0063] Generally, the first DCDC circuit is implemented using a three-level Boost circuit. Refer to Figure 2 , which is a schematic diagram of a first DCDC circuit provided by an embodiment of the present application.

[0064] In an actual product, the output terminals of multiple first DCDC circuits may be connected in parallel, and the input terminals of each first DCDC circuit are respectively connected to different photovoltaic strings. For example, Figure 2 takes three first DCDC circuits as an example for introduction. The input terminal of the first DCDC circuit is connected to PV1, the input terminal of the second DCDC circuit is connected to PV2, and the input terminal of the third DCDC circuit is connected to PV3.

[0065] During normal operation, the clamping diode D will not conduct, but in some transient operating conditions, the clamping diode D will conduct, that is, as shown by the implementation arrow in Figure 2 , a current path is formed, causing the half-bus voltage at the output terminal of the first DCDC circuit to be unbalanced. It should be understood that the half-bus voltage here is relative to half of the output voltage of the first DCDC circuit, which is different from the half-bus voltage between BUS+ and the N line in the photovoltaic system.

[0066] To solve the above technical problems, when the half-bus voltage is unbalanced, the half-bus voltage is balanced.

[0067] To enable those skilled in the art to better understand the solution of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0068] It should be understood that the terms "first", "second", "third", "fourth", etc. in the embodiments of the present application are only for convenience of description and do not constitute a limitation to the present application.

[0069] See Figure 3 , which is a schematic diagram of another photovoltaic system provided by the embodiments of the present application.

[0070] The photovoltaic system provided in this embodiment includes: a first DCDC circuit 10, a second DCDC circuit 20, a first capacitor C1, a second capacitor C2, an energy storage element 50, and a controller (not shown in the figure);

[0071] The positive input terminal and the negative input terminal of the first DCDC circuit 10 are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit 10 is connected to the positive bus BUS+;

[0072] The positive input terminal and the negative input terminal of the second DCDC circuit 20 are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit 10; the positive output terminal of the second DCDC circuit 20 is connected to the N line, and the positive output terminal of the second DCDC circuit 20 is connected to the negative bus BUS-;

[0073] Both ends of the first capacitor C1 are respectively connected to BUS+ and the midpoint of the output terminal of the first DCDC circuit 10, both ends of the second capacitor C2 are respectively connected to the midpoint of the output terminal of the first DCDC circuit 10 and the negative output terminal of the first DCDC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit 10;

[0074] The voltage at the midpoint of the output terminal of the first DCDC circuit 10 may be unbalanced, which may be that the voltage on the first capacitor C1 is greater than the voltage on the second capacitor C2, or the voltage on the second capacitor C2 is greater than the voltage on the first capacitor C1. When the voltage on the first capacitor C1 is equal to the voltage on the second capacitor C2, the voltage at the midpoint of the output terminal of the first DCDC circuit 10 reaches balance.

[0075] It should be understood that the technical problem solved by the photovoltaic system provided in the embodiments of the present application is the balance of the potential at the midpoint of the output terminal of the first DCDC circuit. Therefore, the voltages that need to be detected are the first voltage between the positive output terminal of the first DCDC circuit and the midpoint of the output terminal, and the second voltage between the negative output terminal of the first DCDC circuit and the midpoint of the output terminal. For convenience of description only, the first voltage is referred to as the voltage of the first capacitor C1, and the second voltage is referred to as the voltage on the second capacitor C2.

[0076] A controller, configured to transfer the energy of the first capacitor C1 to the energy storage element 50 when the voltage of the first capacitor C1 is greater than that of the second capacitor C2, and then transfer the energy of the energy storage element 50 to the second capacitor C2; when the voltage of the second capacitor C2 is greater than that of the first capacitor C1, transfer the energy of the second capacitor to the energy storage element 50, and then transfer the energy of the energy storage element 50 to the first capacitor C2.

[0077] In the photovoltaic system provided by the embodiment of the present application, when the voltage on the first capacitor is greater than the voltage on the second capacitor, it indicates that there is an imbalance in the voltage at the midpoint of the output terminal of the first DCDC circuit 10. Therefore, in order to balance the voltage at the midpoint of the output terminal of the first DCDC circuit 10, it is necessary to reduce the voltage on the first capacitor and increase the voltage on the second capacitor. Therefore, an energy storage element is added in the present application. In order not to cause energy loss, the excess energy on the first capacitor is transferred to the energy storage element, and then transferred from the energy storage element to the second capacitor until the voltage on the first capacitor is equal to the voltage on the second capacitor. Similarly, when the voltage on the second capacitor is greater than the voltage on the first capacitor, it is necessary to reduce the voltage on the second capacitor and increase the voltage on the first capacitor. Therefore, the excess energy on the second capacitor is transferred to the energy storage element, and then transferred from the energy storage element to the first capacitor until the voltage of the first capacitor is equal to the voltage of the second capacitor, finally making the voltage at the midpoint of the output terminal of the first DCDC circuit 10 balanced.

[0078] A specific implementation manner will be introduced in detail below with reference to the accompanying drawings.

[0079] See Figure 4 , which is a schematic diagram of another photovoltaic system provided by the embodiment of the present application.

[0080] The photovoltaic system provided by this embodiment further includes: a first switch unit 60 and a second switch unit 70;

[0081] The first end of the first switch unit 60 is connected to BUS+, and the second end of the first switch unit 60 is connected to the midpoint of the output terminal of the first DCDC circuit;

[0082] The first end of the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit, and the second end of the energy storage element is connected to the second end of the first switch unit 60;

[0083] The first end of the second switch unit 70 is connected to the second end of the first switch unit 60, and the second end of the second switch unit 70 is connected to the N line;

[0084] The controller is specifically configured to, when the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, close the first switch unit 60 by control. When the first switch unit 60 is closed, the first capacitor C1, the first switch unit 60, and the energy storage element 50 form a closed loop. At this time, the energy of the first capacitor C1 can be transferred to the energy storage element 50; then the first switch unit 60 is disconnected, and the energy of the energy storage element 50 is transferred to the second capacitor C2, that is, the energy transfer can be realized, the voltage on the first capacitor C1 is decreased, and the voltage on the second capacitor C2 is increased until the voltage on the first capacitor C1 is equal to the voltage on the second capacitor C2.

[0085] The controller is specifically configured to, when the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, close the second switch unit 70 by control. When the second switch unit 70 is closed, the second capacitor C2, the second switch unit 70, and the energy storage element 50 form a closed loop. At this time, the energy on the second capacitor C2 can be transferred to the energy storage element 50; then the second switch unit 70 is disconnected, and the energy of the energy storage element 50 is transferred to the first capacitor C1, thereby realizing the decrease of the voltage of the second capacitor C2 and the increase of the voltage on the first capacitor C1 until the voltage on the first capacitor C1 is equal to the voltage on the second capacitor C2.

[0086] In this embodiment, the energy storage element 50 includes an inductor as an example for introduction.

[0087] The case where both the first switch unit and the second switch unit include one switching tube is introduced below.

[0088] See Figure 5A , which is a schematic diagram of another photovoltaic system provided by the embodiment of the present application.

[0089] For the photovoltaic system provided in this embodiment, the first switch unit includes a first switching tube Q1, and the second switch unit includes a second switching tube Q2;

[0090] Both the first switching tube Q1 and the second switching tube Q2 include anti-parallel diodes;

[0091] The controller (not shown in the figure) is specifically configured to control both the first switching tube Q1 and the second switching tube Q2 to be disconnected when transferring the energy of the energy storage element to the second capacitor C2; and control both the first switching tube Q1 and the second switching tube Q2 to be disconnected when transferring the energy of the energy storage element to the first capacitor C1.

[0092] Since the first switching transistor Q1 and the second switching transistor Q2 provided in the embodiments of the present application both include anti-parallel diodes, when transferring energy from the energy storage element to the capacitor, it is not necessary to control the switching transistors to operate, and a freewheeling circuit can be directly formed by using the diodes anti-parallel to the switching transistors. In this way, the control of the switching transistors is simpler. Of course, the switching transistors can also be controlled to operate to form a path. For example, when transferring energy from the energy storage element 50 to the second capacitor C2, the first switching transistor Q1 can also be controlled to turn off and the second switching transistor Q2 can be controlled to turn on to achieve this. Since the conduction power consumption of the switching transistor is smaller than that of the diode, power consumption can be saved.

[0093] Figure 5A In the above, the case where both the first switching unit and the second switching unit include controllable switching transistors is taken as an example for introduction. In addition, according to the specific application scenario, a mixture of a controllable switching transistor and a diode can also be selected. For example, when only the voltage of the first capacitor is greater than the voltage of the second capacitor in the application scenario, the first switching unit can include a controllable switching transistor and the second switching unit can include a diode. On the contrary, when the voltage of the second capacitor is greater than the voltage of the first capacitor, the first switching unit can include a diode and the second switching unit can include a controllable switching transistor. The following will be introduced in detail with reference to the drawings.

[0094] See Figure 5B , which is a schematic diagram of another photovoltaic system provided by the embodiments of the present application.

[0095] When the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the first switching unit includes the first switching transistor Q1 and the second switching unit includes the second diode D2;

[0096] The controller is specifically configured to control the first switching transistor Q1 to turn on when transferring the energy of the first capacitor C1 to the energy storage element. When transferring the energy of the energy storage element to the second capacitor C2, by using the unidirectional conduction characteristic of the diode, a current path can be formed, and it is not necessary to control the diode to operate, and the freewheeling function can be automatically completed.

[0097] See Figure 5C , which is a schematic diagram of yet another photovoltaic system provided by the embodiments of the present application.

[0098] When the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, the first switching unit includes the first diode D1 and the second switching unit includes the second switching transistor Q2;

[0099] The controller is specifically configured to control the second switching transistor Q2 to turn on when transferring the energy of the second capacitor C2 to the energy storage element. When transferring the energy of the energy storage element to the first capacitor C1, by using the unidirectional conduction characteristic of the diode, a current path can be formed, and it is not necessary to control the diode to operate, and the freewheeling function can be automatically completed.

[0100] The following describes the case where the first switching unit includes two switching tubes and the second switching unit includes two switching tubes in the photovoltaic system provided by the embodiments of the present application.

[0101] Refer to Figure 6 , which is a schematic diagram of another photovoltaic system provided by the embodiments of the present application.

[0102] In the photovoltaic system provided by this embodiment, the first switching unit includes a first switching tube Q1 and a second switching tube Q2 connected in series; the second switching tube includes a third switching tube Q3 and a fourth switching tube Q4 connected in series; both the first switching tube and the second switching tube include anti-parallel diodes; both the third switching tube Q3 and the fourth switching tube Q4 include anti-parallel diodes;

[0103] The controller is specifically configured to control the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 to be all turned off when transferring the energy of the energy storage element to the second capacitor C2; and control the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 to be all turned off when transferring the energy of the energy storage element to the first capacitor C1.

[0104] Since each switching unit includes two switching tubes connected in series, when controlling the action of the switching unit, it is necessary to control the two switching tubes connected in series inside the switching unit to act synchronously. For example, when controlling the first switching unit to close, it is necessary to control the first switching tube Q1 and the second switching tube Q2 to conduct simultaneously; when controlling the first switching unit to turn off, it is necessary to control the first switching tube Q1 and the second switching tube Q2 to turn off simultaneously.

[0105] When the energy on the energy storage element is transferred to the capacitor, the working principle is the same as that introduced in the above embodiments. Since the switching tube includes an anti-parallel diode, it is not necessary to control the switching unit corresponding to the capacitor to close, and the current path can be formed from the anti-parallel diode. The specific working principle will not be elaborated here.

[0106] In the embodiments of the present application, the implementation type of the switching tube is not specifically limited. For example, it can be an IGBT or other switching devices such as MOS. The above is only an example introduced with one switching tube included in the switching unit or two switching tubes included in the switching unit. It should be understood that the switching unit can also include other numbers of switching tubes, or other bridge arms that play a switching role are also possible, which will not be elaborated one by one here.

[0107] In the photovoltaic system provided by the embodiments of the present application, each switching unit includes two switching tubes connected in series. Each switching tube can be a switching tube with a lower withstand voltage because the two switching tubes connected in series can share the voltage, thereby reducing the withstand voltage of each switching tube.

[0108] Since the switch unit includes multiple switch tubes, when the switch tubes are driven to operate, some switch tubes may be turned on first and some switch tubes may be turned on later. In order to prevent the switch tube that is turned on first from being subjected to a large voltage that exceeds its withstand voltage, the photovoltaic system provided in the embodiment of the present application further includes: a first diode D1 and a second diode D2;

[0109] The cathode of the first diode D1 is connected to the common end of the first switch tube Q1 and the second switch tube Q2, and the anode of the first diode D1 is connected to the midpoint of the output end of the first DCDC circuit;

[0110] The anode of the second diode D2 is connected to the common end of the third switch tube Q3 and the fourth switch tube Q4 , and the anode of the second diode D2 is connected to the midpoint of the output end of the first DCDC circuit.

[0111] The first diode D1 and the second diode D2 both function to achieve voltage clamping, thereby preventing the voltage borne by the switch tube from exceeding its withstand voltage range and protecting the safe operation of the switch tube.

[0112] In addition, similar Figures 5A - 5C , Figure 6 Some of the switches in the circuit can also be replaced by diodes. For example, when Q3 and Q4 are switches, Q1 and Q2 can be diodes. Similarly, when Q1 and Q2 are switches, Q3 and Q4 can be diodes. The specific working principle is: Figures 5A - 5C Similar, no further description is given here.

[0113] See also Figure 7 , this figure is a schematic diagram of another photovoltaic system provided in an embodiment of the present application.

[0114] In the photovoltaic system provided in this embodiment, the midpoint of the input terminal of the second DCDC circuit 20 is connected to the anode of the first diode D1, which is equivalent to clamping the midpoint of the output busbar of the first DCDC circuit 10 and the midpoint of the input busbar of the second DCDC circuit 20 to the same potential.

[0115] The photovoltaic systems provided in the above embodiments are all described using a three-level boost circuit as the first DCDC circuit. Due to the aforementioned technical issues with the three-level boost circuit, the clamping diode may be turned on under certain operating conditions, resulting in an unbalanced potential at the midpoint of the output terminal of the first DCDC circuit.

[0116] Method Example

[0117] Based on the photovoltaic system provided in the above embodiment, the embodiment of the present application further provides a midpoint balancing method for the photovoltaic system, which is described in detail below with reference to the accompanying drawings.

[0118] See also Figure 8, This figure is a flowchart of a midpoint balancing method for a photovoltaic system provided by an embodiment of the present application.

[0119] The midpoint balancing method for the photovoltaic system provided by this embodiment, wherein the photovoltaic system includes: a first DCDC circuit, a second DCDC circuit, a first capacitor, a second capacitor, and an energy storage element; the positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-; both ends of the first capacitor are respectively connected to BUS+ and the midpoint of the output terminal of the first DCDC circuit, and both ends of the second capacitor are respectively connected to the midpoint of the output terminal of the first DCDC circuit and the negative output terminal of the first DCDC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit;

[0120] This method includes:

[0121] S801: When the voltage of the first capacitor is greater than the voltage of the second capacitor, transfer the energy of the first capacitor to the energy storage element, and then transfer the energy of the energy storage element to the second capacitor;

[0122] S802: When the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor.

[0123] It should be understood that the technical problem solved by the method provided by the embodiment of the present application is the balance of the midpoint potential of the output terminal of the first DCDC circuit. Therefore, the voltages that need to be detected are the first voltage between the positive output terminal of the first DCDC circuit and the midpoint of the output terminal, and the second voltage between the negative output terminal of the first DCDC circuit and the midpoint of the output terminal. For the convenience of description below, the first voltage is referred to as the voltage of the first capacitor, and the second voltage is referred to as the voltage of the second capacitor.

[0124] In the method provided by the embodiment of the present application, when the voltage on the first capacitor is greater than the voltage on the second capacitor, it indicates that the voltage at the midpoint of the output terminal of the first DCDC circuit is unbalanced. Therefore, in order to balance the voltage at the midpoint of the output terminal of the first DCDC circuit, it is necessary to reduce the voltage on the first capacitor and increase the voltage on the second capacitor. Therefore, the present application adds an energy storage element. In order not to cause energy loss, the excess energy on the first capacitor is transferred to the energy storage element and then transferred from the energy storage element to the second capacitor until the voltage on the first capacitor is equal to the voltage on the second capacitor. Similarly, when the voltage on the second capacitor is greater than the voltage on the first capacitor, it is necessary to reduce the voltage on the second capacitor and increase the voltage on the first capacitor. Therefore, the excess energy on the second capacitor is transferred to the energy storage element and then transferred from the energy storage element to the first capacitor until the voltage of the first capacitor is equal to the voltage of the second capacitor, finally making the voltage at the midpoint of the output terminal of the first DCDC circuit reach balance.

[0125] The photovoltaic system further includes: a first switch unit and a second switch unit; the first end of the first switch unit is connected to BUS+, and the second end of the first switch unit is connected to the midpoint of the output terminal of the first DCDC circuit; the first end of the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit, and the second end of the energy storage element is connected to the second end of the first switch unit; the first end of the second switch unit is connected to the second end of the first switch unit, and the second end of the second switch unit is connected to the N line;

[0126] When the voltage of the first capacitor is greater than the voltage of the second capacitor, transferring the energy of the first capacitor to the energy storage element specifically includes:

[0127] When the voltage of the first capacitor is greater than the voltage of the second capacitor, by controlling the first switch unit to close, the energy of the first capacitor is transferred to the energy storage element;

[0128] When the voltage of the second capacitor is greater than the voltage of the first capacitor, transferring the energy of the second capacitor to the energy storage element specifically includes:

[0129] When the voltage of the second capacitor is greater than the voltage of the first capacitor, by controlling the second switch unit to close, the energy of the second capacitor is transferred to the energy storage element.

[0130] As mentioned above, the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A photovoltaic system, characterized in that, Including: A first DC-DC circuit, a second DC-DC circuit, a first capacitor, a second capacitor, an energy storage element, a first DC-AC circuit, a second DC-AC circuit, and a controller; The positive input terminal and the negative input terminal of the first DC-DC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DC-DC circuit is connected to the positive bus BUS+; The positive input terminal and the negative input terminal of the second DC-DC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DC-DC circuit; the positive output terminal of the second DC-DC circuit is connected to the N line, and the positive output terminal of the second DC-DC circuit is connected to the negative bus BUS-; the N line is used to connect the negative input terminal of the first DC-AC circuit and is connected to the positive input terminal of the second DC-AC circuit; the positive input terminal of the first DC-AC circuit is connected to the BUS+, and the negative input terminal of the second DC-AC circuit is connected to the BUS-; Both ends of the first capacitor are respectively connected to the BUS+ and the midpoint of the output terminal of the first DC-DC circuit, and both ends of the second capacitor are respectively connected to the midpoint of the output terminal of the first DC-DC circuit and the negative output terminal of the first DC-DC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DC-DC circuit; The controller is configured to transfer the energy of the first capacitor to the energy storage element when the voltage of the first capacitor is greater than the voltage of the second capacitor, and then transfer the energy of the energy storage element to the second capacitor; When the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor.

2. The photovoltaic system according to claim 1, wherein Further including: A first switch unit and a second switch unit; The first end of the first switch unit is connected to the BUS+, and the second end of the first switch unit is connected to the midpoint of the output terminal of the first DC-DC circuit; The first end of the energy storage element is connected to the midpoint of the output terminal of the first DC-DC circuit, and the second end of the energy storage element is connected to the second end of the first switch unit; The first end of the second switch unit is connected to the second end of the first switch unit, and the second end of the second switch unit is connected to the N line; The controller is specifically configured to, when the voltage of the first capacitor is greater than the voltage of the second capacitor, close the first switch unit through control to transfer the energy of the first capacitor to the energy storage element; When the voltage of the second capacitor is greater than the voltage of the first capacitor, close the second switch unit through control to transfer the energy of the second capacitor to the energy storage element.

3. The photovoltaic system according to claim 2, wherein, The first switch unit includes a first switch tube, and the second switch unit includes a second switch tube; Both the first switch tube and the second switch tube include anti-parallel diodes; The controller is specifically configured to control both the first switch tube and the second switch tube to be turned off when transferring the energy of the energy storage element to the second capacitor; and control both the first switch tube and the second switch tube to be turned off when transferring the energy of the energy storage element to the first capacitor.

4. The photovoltaic system according to claim 2, wherein The first switching unit includes a first switching tube and a second switching tube connected in series; the second switching tube includes a third switching tube and a fourth switching tube connected in series; both the first switching tube and the second switching tube include anti-parallel diodes; both the third switching tube and the fourth switching tube include anti-parallel diodes; The controller is specifically configured to control the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube to be all turned off when transferring the energy of the energy storage element to the second capacitor; and control the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube to be all turned off when transferring the energy of the energy storage element to the first capacitor.

5. The photovoltaic system according to claim 4, characterized in that It further includes: A first diode and a second diode; The cathode of the first diode is connected to the common terminal of the first switching tube and the second switching tube, and the anode of the first diode is connected to the midpoint of the output terminal of the first DCDC circuit; The anode of the second diode is connected to the common terminal of the third switching tube and the fourth switching tube, and the anode of the second diode is connected to the midpoint of the output terminal of the first DCDC circuit. (There seems to be a mistake here, it should be 'the cathode of the second diode' instead of 'the anode of the second diode' for correct logic. But translating as is for now.) 6. The photovoltaic system according to claim 5, characterized in that The midpoint of the input terminal of the second DCDC circuit is connected to the anode of the first diode.

7. The photovoltaic system according to any one of claims 1-6, characterized in that, The energy storage element includes an inductor.

8. The photovoltaic system according to claim 2, wherein The first switching unit includes a first switching tube, and the second switching unit includes a second diode; The controller is specifically configured to control the first switching tube to be closed when transferring the energy of the first capacitor to the energy storage element when the voltage of the first capacitor is greater than the voltage of the second capacitor.

9. The photovoltaic system according to claim 2, wherein, The first switching unit includes a first diode, and the second switching unit includes a second switching tube; The controller is specifically configured to control the second switching tube to be closed when transferring the energy of the second capacitor to the energy storage element when the voltage of the second capacitor is greater than the voltage of the first capacitor.

10. A midpoint balancing method for a photovoltaic system, characterized in that, The photovoltaic system includes: a first DCDC circuit, a second DCDC circuit, a first capacitor, a second capacitor, and an energy storage element; the positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the N line is used to connect the negative input terminal of the first DCAC circuit and the positive input terminal of the second DCAC circuit; the positive input terminal of the first DCAC circuit is connected to the BUS+; the negative input terminal of the second DCAC circuit is connected to BUS-; the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-; both ends of the first capacitor are respectively connected to the BUS+ and the midpoint of the output terminal of the first DCDC circuit; both ends of the second capacitor are respectively connected to the midpoint of the output terminal of the first DCDC circuit and the negative output terminal of the first DCDC circuit; the energy storage element is connected to the midpoint of the output terminal of the first DCDC circuit; The method includes: When the voltage of the first capacitor is greater than the voltage of the second capacitor, transfer the energy of the first capacitor to the energy storage element, and then transfer the energy of the energy storage element to the second capacitor; When the voltage of the second capacitor is greater than the voltage of the first capacitor, transfer the energy of the second capacitor to the energy storage element, and then transfer the energy of the energy storage element to the first capacitor.

11. The method according to claim 10, wherein, The photovoltaic system further includes: a first switch unit and a second switch unit; a first end of the first switch unit is connected to the BUS+, a second end of the first switch unit is connected to a midpoint of an output end of the first DCDC circuit; a first end of the energy storage element is connected to the midpoint of the output end of the first DCDC circuit, a second end of the energy storage element is connected to the second end of the first switch unit; a first end of the second switch unit is connected to the second end of the first switch unit, and a second end of the second switch unit is connected to the N line; When the voltage of the first capacitor is greater than the voltage of the second capacitor, transferring the energy of the first capacitor to the energy storage element specifically includes: When the voltage of the first capacitor is greater than the voltage of the second capacitor, by controlling the first switch unit to close, transfer the energy of the first capacitor to the energy storage element; When the voltage of the second capacitor is greater than the voltage of the first capacitor, transferring the energy of the second capacitor to the energy storage element specifically includes: When the voltage of the second capacitor is greater than the voltage of the first capacitor, by controlling the second switch unit to close, transfer the energy of the second capacitor to the energy storage element.

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