Hybrid energy storage system with bidirectional three-level and neutral point voltage balancing method thereof

Abstract

The present invention discloses a hybrid energy storage system with bidirectional three-level and a neutral point voltage balancing method thereof, comprising a three-level conversion circuit and a hybrid energy storage device, wherein the hybrid energy storage device comprises a super capacitor, a battery control circuit and a battery; one side of the three-level conversion circuit is connected in parallel with the DC side, and the other side is connected in parallel with the super capacitor unit, the battery control circuit and the battery unit in sequence; the three-level conversion circuit is used to convert the input level of the DC side into a three-level input hybrid energy storage device, or convert the output level of the hybrid energy storage device into a three-level output from the DC side; the battery control circuit is used to control the charging and discharging of the battery unit. Two energy storage devices share a three-level converter, which reduces the volume and cost of the entire hybrid energy storage system, selects different working voltage levels according to the current requirements, and realizes independent control of the charging and discharging of the super capacitor and the battery. This circuit can also balance the neutral point voltage using the three-level characteristics.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrids, and relates to a hybrid energy storage system with two-way three-levels and a neutral point voltage balancing method thereof. Background Art

[0002] In order to improve the transient and steady-state performance of the DC microgrid system, a hybrid energy storage device is introduced into the DC microgrid, that is, combining two energy storage devices with complementary characteristics. Currently, the most commonly used devices for hybrid energy storage are batteries and supercapacitors.

[0003] In traditional hybrid energy storage devices, each energy storage device must be connected to a converter separately. This structure will greatly increase the volume and cost of the hybrid energy storage, and at the same time make the control strategy complicated. Because the voltage difference between the DC bus voltage and its terminal voltage is large when the energy storage device is in a low state of charge, a large inductor is required to reduce the fluctuation of the charging and discharging current of the energy storage device, which will also increase the volume of the hybrid energy storage.

[0004] At present, a new battery-supercapacitor hybrid energy storage circuit structure has been proposed for electric vehicles. This structure reduces the number of power semiconductors, but it only has two voltage levels. The supercapacitor cannot effectively suppress high-frequency DC voltage fluctuations, which will lead to deterioration of DC bus voltage regulation. Therefore, this circuit structure is not suitable for application in microgrids; others have proposed another new hybrid energy storage system configuration that can partially alleviate the high voltage and high-frequency ripples on supercapacitors. However, batteries and supercapacitors cannot be controlled independently, because DC microgrids require independent regulation and proper coordination between different energy storage devices, so this circuit structure cannot meet the needs of microgrids. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the hybrid energy storage device in the above-mentioned prior art, such as large size, high cost and only two voltage levels, and to provide a bidirectional three-level hybrid energy storage system and a neutral point voltage balancing method thereof.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] In one aspect of the present invention, a bidirectional three-level hybrid energy storage system includes a three-level conversion circuit and a hybrid energy storage device, wherein the hybrid energy storage device includes a super capacitor unit, a battery control circuit and a battery unit; one side of the three-level conversion circuit is connected in parallel to a DC side, and the other side is connected in parallel to the super capacitor unit, the battery control circuit and the battery unit in sequence;

[0008] The three-level conversion circuit is used to convert the input level of the DC side into a three-level input hybrid energy storage device, or convert the output level of the hybrid energy storage device into a three-level output from the DC side;

[0009] The battery control circuit is used to control the charging and discharging of the battery unit.

[0010] The present invention is further improved in that:

[0011] The three-level conversion circuit includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a first energy storage capacitor and a second energy storage capacitor;

[0012] One end of the first energy storage capacitor is connected to one end of the second energy storage capacitor, and the other end is connected to the DC side positive bus and the collector of the first switch tube; the other end of the second energy storage capacitor is connected to the DC side negative bus; the emitter of the first switch tube is connected to the collector of the second switch tube, the connecting line between the first switch tube and the second switch tube is connected to the positive end of the supercapacitor unit, the emitter of the second switch tube is connected to the collector of the third switch tube, the emitter of the third switch tube is connected to the collector of the fourth switch tube and the negative end of the supercapacitor unit, the emitter of the fourth switch tube is connected to one end of the second energy storage capacitor connected to the DC side negative bus, a neutral point N is set on the connecting line of the first energy storage capacitor and the second energy storage capacitor, and the neutral point N is connected to the connecting line between the second switch tube and the third switch tube.

[0013] The level conversion circuit further includes a filter inductor, and the connection line between the first switch tube and the second switch tube is connected to the positive terminal of the super capacitor unit through the filter inductor.

[0014] The first switch tube, the second switch tube, the third switch tube and the fourth switch tube are all IGBT switch tubes.

[0015] The battery control circuit includes a first switch tube and a second switch tube; the collector of the first switch tube is connected to the positive terminal of the supercapacitor unit, the emitter is connected to the collector of the second switch tube, the emitter of the second switch tube is connected to the negative terminal of the supercapacitor unit and the negative terminal of the battery unit, and the connecting line between the first switch tube and the second switch tube is connected to the positive terminal of the battery unit.

[0016] The battery control circuit also includes a filter inductor, and the connection line between the first switch tube and the second switch tube is connected to the positive terminal of the battery unit through the filter inductor.

[0017] The first switch tube and the second switch tube are both IGBT switch tubes.

[0018] The supercapacitor unit includes a plurality of supercapacitors connected in series, and the battery unit includes a plurality of batteries connected in series.

[0019] In another aspect of the present invention, a neutral point voltage balancing method of a bidirectional three-level hybrid energy storage system comprises the following steps:

[0020] S1: ΔV is obtained by equation (1)c :

[0021] ΔV c =V c1 -V c2 (1)

[0022] Among them, V c1 is the voltage of the first energy storage capacitor, V c2 is the voltage of the second energy storage capacitor;

[0023] S2: When ΔV c ≠0, the first switch tube and the third switch tube are turned on, and the second switch tube and the fourth switch tube are turned off; or the first switch tube and the third switch tube are turned off, and the second switch tube and the fourth switch tube are turned on;

[0024] S3: until ΔV c =0, completing the neutral point voltage balance adjustment.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention has a bidirectional three-level hybrid energy storage system, which connects two energy storage devices, a supercapacitor unit and a battery unit, through a three-level conversion circuit. Compared with the design in which each energy storage device in the traditional hybrid energy storage system is individually connected to a converter, the volume and cost of the hybrid energy storage system can be greatly reduced, and the control strategy can also be simplified; the design of the three-level conversion circuit can realize three levels of input and output. When the energy storage device is in a low state of charge, when the voltage difference between the DC side voltage and the hybrid energy storage device voltage is large, the voltage difference between the DC side voltage and the hybrid energy storage device voltage can be reduced by an intermediate level, and a large inductance is not required to reduce the fluctuation of the charging and discharging current of the hybrid energy storage device, further reducing the volume and cost of the hybrid energy storage device. At the same time, a battery control circuit is set, and the battery charging and discharging is controlled by the battery control circuit to realize independent control of the two energy storage devices, the supercapacitor unit and the battery unit, and the battery unit and the supercapacitor unit are independently controlled, thereby better coordinating the energy distribution of the battery unit and the supercapacitor unit.

[0027] The neutral point voltage balancing method of the present invention realizes voltage balance of the two capacitors by turning on the first switch tube and the third switch tube and disconnecting the second switch tube and the fourth switch tube; or disconnecting the first switch tube and the third switch tube and turning on the second switch tube and the fourth switch tube; discharging the larger capacitor to reduce the discharge, or charging the smaller capacitor to increase the charge, when the voltages of the two capacitors are unbalanced. The neutral point voltage fluctuation caused by the voltage imbalance of the upper and lower capacitors can be effectively suppressed, thereby ensuring the accuracy of the neutral point voltage value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1A topological diagram of a bidirectional three-level hybrid energy storage system according to the present invention;

[0029] Figure 2 A schematic diagram of charging at a Vdc level according to the present invention;

[0030] Figure 3 A schematic diagram of a charging method at a Vdc / 2 level according to the present invention;

[0031] Figure 4 A schematic diagram of another charging method at a Vdc / 2 level according to the present invention;

[0032] Figure 5 The figure is a flow chart of the neutral point voltage balancing method of the present invention.

[0033] Among them: 1- three-level conversion circuit; 2- super capacitor; 3- battery control circuit; 4- battery. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0035] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0036] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0037] See also Figure 1The present invention has a bidirectional three-level hybrid energy storage system, including a three-level conversion circuit 1 and a hybrid energy storage device; the three-level conversion circuit 1 is used to convert the input level of the DC side into a three-level input hybrid energy storage device, or convert the output level of the hybrid energy storage device into a three-level; the hybrid energy storage device includes a super capacitor 2, a battery control circuit 3 and a battery 4; one side of the three-level conversion circuit 1 is connected in parallel with the super capacitor 2, the battery control circuit 3 and the battery 4 in sequence, and the battery control circuit 3 is used to control the charge and discharge of the battery 4. The other side of the three-level conversion circuit 1 is connected in parallel with the DC side, and the DC side includes a DC side negative bus and a DC side positive bus.

[0038] The three-level conversion circuit 1 includes four IGBT switches, namely a first switch S1 , a second switch S2 , a third switch S3 and a fourth switch S4 , a first energy storage capacitor C1 and a second energy storage capacitor C2 , and also includes a first filter inductor L1 .

[0039] One end of the first energy storage capacitor C1 is connected to one end of the second energy storage capacitor C2, and the other end is connected to the DC side positive bus and the collector of the first switch tube S1; the other end of the second energy storage capacitor C2 is connected to the DC side negative bus; the emitter of the first switch tube S1 is connected to the collector of the second switch tube S2, the connecting line between the first switch tube S1 and the second switch tube S2 is connected to the positive end of the super capacitor 2 through the first filter inductor L1, the emitter of the second switch tube S2 is connected to the collector of the third switch tube S3, the emitter of the third switch tube S3 is connected to the collector of the fourth switch tube S4 and the negative end of the super capacitor 2, the emitter of the fourth switch tube S4 is connected to one end of the second energy storage capacitor C2 connected to the DC side negative bus, a neutral point N is set on the connecting line of the first energy storage capacitor C1 and the second energy storage capacitor C2, and the neutral point N is connected to the connecting line between the second switch tube S2 and the third switch tube S3.

[0040] The battery control circuit 3 includes two IGBTs, namely a fifth switch tube S5 and a sixth switch tube S6, and a second filter inductor L2.

[0041] The collector of the fifth switch tube S5 is connected to the positive terminal of the supercapacitor 2, and the emitter is connected to the collector of the sixth switch tube S6. The emitter of the sixth switch tube S6 is connected to the negative terminal of the supercapacitor 2 and the negative terminal of the battery 4. The connecting line between the fifth switch tube S5 and the sixth switch tube S6 is connected to the positive terminal of the battery 4 through the second filter inductor L2.

[0042] The supercapacitor 2 may be replaced by a supercapacitor unit formed by connecting a plurality of supercapacitors 2 in series and in parallel, and the storage battery 4 may be replaced by a storage battery unit formed by connecting a plurality of storage batteries 4 in series and in parallel.

[0043] In this embodiment, the DC bus voltage is 50V, the first energy storage capacitor C1 and the second energy storage capacitor C2 are both 440μf, the first filter inductor L1 is 1mH, the second filter inductor L1 is 0.5mH, the super capacitor 2 is 29F, the IGBT switch tube is IRGB4065, the operating voltage is 300V, the operating current is 70A, the battery is IP5642128-4S1P lithium battery, the charging voltage is 16.8V, and the charging current is less than 2.1A.

[0044] The principle of the present invention is described below:

[0045] See also Figures 2 to 4 When the DC side level selects Vdc, the first switch tube S1 and the fourth switch tube S4 are closed, the second switch tube S2 and the third switch tube S3 are disconnected, and the first filter inductor L1, the first switch tube S1, the first energy storage capacitor C1, the second energy storage capacitor C2 and the fourth switch tube S4 are connected in series.

[0046] When the DC side level selects Vdc / 2, the first switch tube S1 and the third switch tube S3 are closed, the second switch tube S2 and the fourth switch tube S4 are disconnected, and the first filter inductor L1, the first switch tube S1, the first energy storage capacitor C1 and the third switch tube S3 are connected in series; or the first switch tube S1 and the third switch tube S3 are disconnected, the second switch tube S2 and the fourth switch tube S4 are closed, and the first filter inductor L1, the second switch tube S2, the second energy storage capacitor C2 and the fourth switch tube S4 are connected in series.

[0047] Among them, the black bold devices in the figure are devices in working state in this working mode, and the arrows indicate the direction of current flow. When the hybrid energy storage device is discharged, the current direction is opposite to the current direction in the figure.

[0048] When the battery 4 is charging, the fifth switch tube S5 is closed, the sixth switch tube S6 is opened, and the fifth switch tube S5 is connected in series with the second filter inductor L2; when the battery 4 is discharging, the fifth switch tube S5 is opened, the sixth switch tube S6 is closed, and the sixth switch tube S6 is connected in series with the second filter inductor L2.

[0049] Basis for selecting the level: Select the charge and discharge level of the energy storage system based on the slope of the current flowing through the inductor.

[0050] The formula for calculating the slope of the specified inductor current is:

[0051] To ensure a continuous and smooth inductor current, |V L -V uc | is small enough to be realized by the three-level characteristics of the DC / DC converter, that is, when When (1) select At this time, the first switch tube S1 and the third switch tube S3 are working, and the super capacitor 2 is charged; (2) Select V L =0, at this time the first switch tube S1 and the third switch tube S3 are working. When selecting V L =V dc At this time, the second switch tube S2 and the fourth switch tube S4 are working.

[0052] During the circuit operation, it is assumed that V C1 =V C2 =Vdc / 2, where V C1 is the voltage of the first energy storage capacitor C1, V C2 is the voltage of the second energy storage capacitor C2, that is, the voltage at the neutral point is always balanced, but in actual work, this assumption is not valid, V C1 and V C2 The value of may vary greatly, so the neutral point voltage fluctuation is formed, which will reduce the current response of the energy storage device. Therefore, a neutral point voltage fluctuation balancing solution must be used to effectively balance V C1 and V C2 The present invention also discloses a neutral point voltage balancing method for a bidirectional three-level hybrid energy storage system, see Figure 5 , specifically including the following steps:

[0053] (1) When When, due to No neutral point current is generated, and the full DC bus voltage V dc , the neutral point voltage will not be affected, where V dc is the DC side voltage, V uc is the voltage of supercapacitor 2.

[0054] (2) When When ΔV c =V c1 -V c2 , where V c1 is the voltage of the first energy storage capacitor C1, V c2 is the voltage of the second energy storage capacitor C2.

[0055] When ΔV c >0, the first energy storage capacitor C1 can be discharged to reduce V C1 Or by charging the second energy storage capacitor C2 to increase V C2 So that |ΔV c |Reduce to 0 to achieve neutral point voltage balance regulation.

[0056] When the first energy storage capacitor C1 is discharged to reduce V C1 So that |ΔV cWhen | decreases, i L1 >0, the energy storage device is charged, the first switch tube S1 and the third switch tube S3 are turned on, the second switch tube S2 and the fourth switch tube S4 are turned off, until |ΔV c |=0; where i L1 is the first filter inductor current, and the current direction flowing from the three-level control circuit 1 to the hybrid energy storage device is defined as positive. When the second energy storage capacitor C2 is charged to increase V C2 So that |ΔV c When | decreases, i L1 <0, the hybrid energy storage device discharges, the first switch tube S1 and the third switch tube S3 are turned off, the second switch tube S2 and the fourth switch tube S4 are turned on, until |ΔV c |=0.

[0057] When ΔV c <0, the first energy storage capacitor C1 can be charged to increase V C1 Or discharge the second energy storage capacitor C2 to reduce V C2 So that |ΔV c |Reduce to 0 to achieve neutral point voltage balance regulation.

[0058] When the first energy storage capacitor C1 is charged to increase V C1 So that |ΔV c When | decreases, i L1 <0, the hybrid energy storage device discharges, the first switch tube S1 and the third switch tube S3 are turned on, and the second switch tube S2 and the fourth switch tube S4 are turned off; to |ΔV c |=0; when the second energy storage capacitor C2 is discharged to reduce V C2 So that |ΔV c When | decreases, i L1 >0, the hybrid energy storage device is charged, the first switch tube S1 and the third switch tube S3 are turned off, the second switch tube S2 and the fourth switch tube S4 are turned on, until |ΔV c |=0.

[0059] The two energy storage devices, the supercapacitor unit and the battery unit, are connected through a three-level conversion circuit. Compared with the design in which each energy storage device in the traditional hybrid energy storage system is connected to a converter separately, the volume and cost of the hybrid energy storage system can be greatly reduced, and the control strategy can be simplified; the design of the three-level conversion circuit can realize three levels of input and output. When the energy storage device is in a low state of charge, when the voltage difference between the DC side voltage and the hybrid energy storage device voltage is large, the intermediate level can be used to reduce the voltage difference between the DC side voltage and the hybrid energy storage device voltage, and no large inductance is required to reduce the fluctuation of the charging and discharging current of the hybrid energy storage device, further reducing the volume and cost of the hybrid energy storage device. At the same time, a battery control circuit is set, and the battery charging and discharging is controlled by the battery control circuit to realize the independent control of the two energy storage devices, the supercapacitor unit and the battery unit, and the battery unit and the supercapacitor unit are independently controlled, so as to better coordinate the energy distribution of the battery unit and the supercapacitor unit.

[0060] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A bidirectional three-level hybrid energy storage system, characterized in that: The invention comprises a three-level conversion circuit (1) and a hybrid energy storage device, wherein the hybrid energy storage device comprises a super capacitor unit, a battery control circuit (3) and a battery unit; one side of the three-level conversion circuit (1) is connected in parallel with a DC side, and the other side is connected in parallel with the super capacitor unit, the battery control circuit (3) and the battery unit in sequence; The three-level conversion circuit (1) is used to convert the input level of the DC side into a three-level input hybrid energy storage device, or to convert the output level of the hybrid energy storage device into a three-level output from the DC side; A battery control circuit (3), used to control the charging and discharging of the battery unit; The three-level conversion circuit (1) comprises a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a first energy storage capacitor and a second energy storage capacitor; One end of the first energy storage capacitor is connected to one end of the second energy storage capacitor, and the other end is connected to the DC side positive bus and the collector of the first switch tube; the other end of the second energy storage capacitor is connected to the DC side negative bus; the emitter of the first switch tube is connected to the collector of the second switch tube, the connecting line of the first switch tube and the second switch tube is connected to the positive end of the supercapacitor unit, the emitter of the second switch tube is connected to the collector of the third switch tube, the emitter of the third switch tube is connected to the collector of the fourth switch tube and the negative end of the supercapacitor unit, the emitter of the fourth switch tube is connected to one end of the second energy storage capacitor connected to the DC side negative bus, a neutral point N is set on the connecting line of the first energy storage capacitor and the second energy storage capacitor, and the neutral point N is connected to the connecting line of the second switch tube and the third switch tube; The battery control circuit (3) comprises a first switch tube and a second switch tube; The collector of the first switch tube is connected to the positive terminal of the supercapacitor unit, the emitter is connected to the collector of the second switch tube, the emitter of the second switch tube is connected to the negative terminal of the supercapacitor unit and the negative terminal of the battery unit, and the connecting line between the first switch tube and the second switch tube is connected to the positive terminal of the battery unit; The battery control circuit (3) also includes a filter inductor, and the connection line between the first switch tube and the second switch tube is connected to the positive terminal of the battery unit through the filter inductor.

2. The bidirectional three-level hybrid energy storage system according to claim 1, characterized in that: The level conversion circuit (1) further comprises a filter inductor, and the connection line between the first switch tube and the second switch tube is connected to the positive terminal of the super capacitor unit via the filter inductor.

3. The bidirectional three-level hybrid energy storage system according to claim 1, characterized in that: The first switch tube, the second switch tube, the third switch tube and the fourth switch tube are all IGBT switch tubes.

4. The bidirectional three-level hybrid energy storage system according to claim 1, characterized in that: The first switch tube and the second switch tube are both IGBT switch tubes.

5. The bidirectional three-level hybrid energy storage system according to claim 1, characterized in that: The supercapacitor unit comprises a plurality of supercapacitors (2) connected in series, and the battery unit comprises a plurality of batteries (4) connected in series.

6. A neutral point voltage balancing method of a bidirectional three-level hybrid energy storage system according to claim 1, characterized in that: The following steps are involved: S1: obtained by formula (1) : in, is the voltage of the first energy storage capacitor, is the voltage of the second energy storage capacitor; S2: When When the first switch tube and the third switch tube are turned on, the second switch tube and the fourth switch tube are turned off; or the first switch tube and the third switch tube are turned off, and the second switch tube and the fourth switch tube are turned on; S3: Until , complete the neutral point voltage balance adjustment.

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