Power system and circulating current suppression method therefor

Abstract

The present application provides a circulating current suppression method, which is applied to a power system having a plurality of power supply modules, each power supply module comprising a high-voltage bus, a low-voltage bus, and a balancing circuit provided with a neutral voltage. The circulating current suppression method comprises: in each balancing circuit, providing a first capacitor electrically coupled between the high-voltage bus and the neutral voltage, and providing a second capacitor electrically coupled between the neutral voltage and the low-voltage bus; obtaining an inputted current effective value of each power supply module; if it is detected that the current effective value of at least one power supply module is not maintained at a current reference value, determining that a circulating current is occurring in the at least one power supply module; and operating the balancing circuit of the power supply module in which the circulating current is occurring, enabling the first capacitor or the second capacitor to be charged, so as to adjust the neutral voltage to suppress the circulating current.

Description

Power system and circulating current suppression method thereof Technical Field

[0001] This case involves an electric power system and a circulating current suppression method thereof, and in particular, an electric power system and a circulating current suppression method thereof that suppresses circulating current by controlling the voltage difference between positive and negative DC bus capacitors. Background Art

[0002] In applications where multiple modules are connected in parallel, module size is crucial due to the large number of modules connected in parallel. To minimize module size, the front-stage AC / DC topology often uses a unidirectional, multi-level converter, with the negative output terminal of the back-stage DC / DC directly connected to the negative terminal of the DC bus. According to the aforementioned circuit architecture, when the DC bus voltages of the modules differ, circulating DC current will be generated. This circulating current reduces power conversion efficiency. Because the front-stage AC / DC topology is unidirectional, this circulating current can cause many harmonics in the input current and affect the total harmonic distortion (THD) of the input current. Furthermore, excessive circulating current can cause the neutral point potential of the multi-level converter to shift, affecting the normal operation of the entire system.

[0003] Therefore, it is an urgent need to develop a power system and a circulating current suppression method thereof that can improve the above-mentioned prior art.

[0004] Summary of the Invention

[0005] The purpose of this case is to provide an electric power system and a method for suppressing circulating current therewith, wherein the electric power system has multiple power modules. When circulating current occurs in any power module, this case operates the balancing circuit in the power module to adjust the neutral voltage of the balancing circuit to suppress the circulating current.

[0006] To achieve the aforementioned objectives, the present invention provides a circulating current suppression method for use in a power system having multiple power modules, each of which includes a high-voltage bus, a low-voltage bus, and a balancing circuit having a neutral voltage. The circulating current suppression method includes: disposing a first capacitor in each balancing circuit electrically coupled between the high-voltage bus and the neutral voltage, and disposing a second capacitor electrically coupled between the neutral voltage and the low-voltage bus; obtaining the effective current value of the input of each power module; determining that a circulating current occurs in the at least one power module if the effective current value of at least one power module is not maintained at a current reference value; and operating the balancing circuit of the power module experiencing the circulating current to charge the first capacitor or the second capacitor, thereby adjusting the neutral voltage to suppress the circulating current.

[0007] To achieve the above objectives, the present invention further provides a power system having multiple power modules, wherein the input of each power module receives the same AC input, and each power module includes a DC power bus, a balancing circuit, and a controller. The DC power bus includes a high-voltage bus and a low-voltage bus. The balancing circuit includes a first capacitor, a second capacitor, a first switch, a second switch, and an energy storage inductor. The negative terminal of the first capacitor is connected to the positive terminal of the second capacitor, the positive terminal of the first capacitor is electrically connected to the high-voltage bus, and the negative terminal of the second capacitor is electrically connected to the low-voltage bus. The first terminal of the first switch is electrically connected to the high-voltage bus, the second terminal of the first switch is connected to the first terminal of the second switch, and the second terminal of the second switch is electrically connected to the low-voltage bus. The first terminal of the energy storage inductor is connected to the negative terminal of the first capacitor and the positive terminal of the second capacitor, and the second terminal of the energy storage inductor is connected to the second terminal of the first switch and the first terminal of the second switch. The controller obtains the effective value of the input current of each power module. If the controller detects that the effective value of the current of at least one power module is greater than or less than a current reference value, it determines that a circulating current occurs in the at least one power module. The controller operates the first switch and / or the second switch of the balancing circuit of the power module where the circulating current occurs, so that the energy storage inductor generates an inductive current, and the current direction of the inductive current is opposite to the circulating current. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG1 is a schematic diagram of a circuit structure of a power system according to an embodiment of the present invention;

[0009] FIG2 is a schematic diagram of a circuit structure of a specific embodiment of the power system of FIG1 ;

[0010] FIG3 is a flow chart of a method for suppressing circulating current in a power system according to an embodiment of the present invention;

[0011] FIG4 illustrates the control strategy of the circulating current suppression method of the power system in this case;

[0012] FIG5 is a simulation diagram of a current waveform when a circulating current occurs in a power module without performing circulating current suppression;

[0013] FIG6 is a current waveform simulation diagram when the circulating current suppression method of this case is adopted when a circulating current occurs in the power module;

[0014] FIG7 is a simulation diagram of a capacitor voltage waveform when a circulating current occurs in a power module and the circulating current suppression method of this case is adopted.

[0015] Description of Figure Numbers:

[0016] 1: Power system

[0017] 10: Power module

[0018] 2: AC input source

[0019] 11: Balanced circuit

[0020] 12: Controller

[0021] 13: High voltage bus

[0022] 14: Low voltage bus

[0023] C1: first capacitor

[0024] C2: Second capacitor

[0025] S1: First switch

[0026] S2: Second switch

[0027] L energy storage inductor

[0028] Vn: Neutral voltage

[0029] iz: effective current value

[0030] i: current

[0031] iref: current reference value

[0032] ST1, ST2, ST3, ST4: Steps

[0033] Vc: compensation voltage

[0034] Vd: capacitor voltage difference

[0035] 15: Conversion circuit

[0036] 16: DC-DC converter

[0037] 3: Load

[0038] iLref: reference value of inductor current

[0039] iL: actual value of inductor current

[0040] VC1: voltage on the first capacitor C1

[0041] VC2: voltage on the second capacitor C2 DETAILED DESCRIPTION

[0042] Some typical embodiments that embody the features and advantages of this invention will be described in detail in the following description. It should be understood that this invention is capable of various variations in different embodiments without departing from the scope of this invention, and that the description and drawings herein are intended to be illustrative in nature and not to limit this invention.

[0043] Figure 1 is a schematic diagram of the circuit architecture of a power system according to one embodiment of the present invention. As shown in Figure 1 , the power system 1 includes multiple power modules 10, where the number of power modules 10 is an integer greater than or equal to two. Each power module 10 receives the same AC input source 2 at its input, and each power module 10 outputs electrically coupled to the same load. Generally, circulating current occurs when multiple power modules 10 are connected in parallel, but the present invention is not limited thereto.

[0044] In the power system 1, each power module 10 includes a DC power bus, a balancing circuit 11, and a controller 12. In this embodiment, the AC input source 2 provides three-phase AC power, but this is not limiting. In another embodiment, the AC input source 2 may also provide single-phase AC power.

[0045] In each power module 10, the DC power bus includes a high-voltage bus 13 and a low-voltage bus 14, and the balancing circuit 11 includes a first capacitor C1, a second capacitor C2, a first switch S1, a second switch S2, and an energy storage inductor L. The positive terminal of the first capacitor C1 is electrically connected to the high-voltage bus 13, the negative terminal of the first capacitor C1 is connected to the positive terminal of the second capacitor C2, and the negative terminal of the second capacitor C2 is electrically connected to the low-voltage bus 14. The first capacitor C1 and the second capacitor C2 can be considered as the positive DC bus capacitor and the negative DC bus capacitor, respectively. The first terminal of the first switch S1 is electrically connected to the high-voltage bus 13, the second terminal of the first switch S1 is connected to the first terminal of the second switch S2, and the second terminal of the second switch S2 is electrically connected to the low-voltage bus 14. The first terminal of the energy storage inductor L is connected to the negative terminal of the first capacitor C1 and the positive terminal of the second capacitor C2, and the second terminal of the energy storage inductor L is connected to the second terminal of the first switch S1 and the first terminal of the second switch S2. In addition, the balancing circuit 11 has a neutral voltage Vn, wherein the neutral voltage Vn is coupled to the negative terminal of the first capacitor C1 , the positive terminal of the second capacitor C2 , and the first terminal of the energy storage inductor L.

[0046] In Figure 1, when the power system 1 operates ideally, the low-voltage bus 14 of each power module 10 maintains the same base voltage, and no circulating current occurs. If any of the multiple power modules 10 experiences an abnormality, the low-voltage bus 14 will no longer maintain the base voltage, causing a circulating current in the power system 1 to flow into the abnormal power module 10. Typically, the voltage of the low-voltage bus 14 of the abnormal power module 10 is lower than the base voltage.

[0047] The controller 12 obtains the effective current value iz of the input of each power module 10, where the effective current value iz can be, for example, but not limited to, the average value or the root mean square value of the current i of the input of the power module 10. If the controller 12 detects that the effective current value iz of at least one power module 10 is greater than or less than a current reference value iref, the controller 12 determines that a circulating current is occurring in the at least one power module 10. Accordingly, the controller 12 operates the first switch S1 and the second switch S2 of the balancing circuit 11 of the power module 10 experiencing the circulating current, causing the energy storage inductor L to generate an inductor current, thereby charging the first capacitor C1 or the second capacitor C2, thereby adjusting the neutral voltage Vn to offset the effects of the circulating current, thereby suppressing the circulating current. The inductor current has a current direction opposite to the circulating current.

[0048] In some embodiments, each power module 10 further includes a conversion circuit 15 that receives an AC input source 2 and outputs DC power to a high-voltage bus 13 and a low-voltage bus 14 of the balancing circuit 11. The conversion circuit 15 may be, for example, but not limited to, a Vienna converter (as shown in FIG. 2 ), and is configured to perform power factor correction on the AC input source 2. In some embodiments, each power module 10 further includes a DC-DC converter 16 that receives DC power via the high-voltage bus 13 and the low-voltage bus 14. The DC-DC converter 16 may be, for example, but not limited to, a flying capacitor converter (FCC), as shown in FIG. 2 . The balancing circuit 11 is electrically coupled to the output of the conversion circuit 15 and the input of the DC-DC converter 16 via the high-voltage bus 13 and the low-voltage bus 14. Furthermore, the output of each power module 10 is connected to the same load 3.

[0049] Please refer to Figures 1 and 3. Figure 3 is a flow chart of a method for suppressing circulating current in a power system according to one embodiment of the present invention. This method is applicable to the power system 1 shown in Figure 1 and is executed by the controller 12 of the power system 1. As shown in Figure 3, first, in step ST1, a first capacitor C1 and a second capacitor C2 are set in each balancing circuit 11. Next, in step ST2, the effective current value iz of the input of each power module 10 is obtained. Then, in step ST3, it is detected whether the effective current value iz of each power module 10 is maintained at the current reference value iref. If the result of step ST3 is no, that is, the effective current value iz of at least one power module 10 is not maintained at the current reference value iref, it is determined that a circulating current occurs in the at least one power module 10. At this time, step ST4 is executed to operate the balancing circuit 11 of the power module 10 where the circulating current occurs, so that the first capacitor C1 or the second capacitor C2 is charged to adjust the neutral voltage Vn to suppress the circulating current. On the contrary, if the determination result of step ST3 is yes, that is, the effective current values ​​iz of all power modules 10 are maintained at the current reference value iref, step ST2 is executed again.

[0050] In some embodiments, when no circulating current occurs in each power module 10, that is, when the effective current value iz of each power module 10 is maintained at the current reference value iref, the low-voltage bus 14 of each power module 10 is maintained at the same reference voltage. In some embodiments, when circulating current occurs in the power module 10, the neutral voltage Vn is regulated to increase the voltage of the low-voltage bus 14 by charging the first capacitor C1 or the second capacitor C2 in the power module 10.

[0051] Please refer to Figures 1 and 4. Figure 4 illustrates the control strategy of the circulating current suppression method of the power system in this case. It should be noted that the control of the power system 1 mentioned in this case is all performed by the controller 12 and will not be repeated later. As shown in Figure 4, when there is a circulating current in at least one power module 10, the power module 10 where the circulating current occurs is marked as an abnormal power module, and the difference between the effective current value iz and the current reference value iref of the abnormal power module is obtained, and the compensation voltage Vc is obtained based on the difference. At the same time, the capacitor voltage difference Vd between the second capacitor C2 and the first capacitor C1 in the abnormal power module is obtained, where the capacitor voltage difference Vd is equal to the voltage on the second capacitor C2 minus the voltage on the first capacitor C1.

[0052] By comparing the compensation voltage Vc and the capacitor voltage difference Vd, the direction of the circulating current can be determined, and the direction of the inductor current and the capacitor to be charged can be determined. Specifically, if the compensation voltage Vc is greater than the capacitor voltage difference Vd, the circulating current is determined to be flowing from the input of the abnormal power module to the abnormal power module, and the first switch S1 and / or the second switch S2 are operated to charge the second capacitor C2. At this time, the inductor current of the energy storage inductor L flows from the second end of the energy storage inductor L to the first end of the energy storage inductor L. Conversely, if the compensation voltage Vc is less than the capacitor voltage difference Vd, the circulating current is determined to be flowing from the output of the abnormal power module to the abnormal power module, and the first switch S1 and / or the second switch S2 are operated to charge the first capacitor C1. At this time, the inductor current of the energy storage inductor L flows from the first end of the energy storage inductor L to the second end of the energy storage inductor L. In some embodiments, a reference value iLref of the inductor current is obtained based on the difference between the compensation voltage Vc and the capacitor voltage difference Vd. This reference value iLref is then compared with the actual inductor current iL, and control signals for the first and second switches S1 and S2 are generated based on the comparison result. In this way, the charging time for the first and second capacitors C1 and C2 can be determined based on the difference between the compensation voltage Vc and the capacitor voltage difference Vd, and the first and / or second switches S1 and S2 can be operated accordingly.

[0053] Furthermore, after the first capacitor C1 or the second capacitor C2 in the power module 10 where the circulating current occurs is charged, the effective current value iz of the power module 10 is again checked to see if it remains at the current reference value iref. If so, the operation of the balancing circuit 11 of the power module 10 is stopped. If the effective current value iz still does not remain at the current reference value iref, the balancing circuit 11 is operated again to charge the first capacitor C1 or the second capacitor C2.

[0054] FIG5 is a current waveform simulation diagram when a circulating current occurs in the power module without performing circulating current suppression, FIG6 is a current waveform simulation diagram when the circulating current suppression method of the present invention is adopted when a circulating current occurs in the power module, and FIG7 is a capacitor voltage waveform simulation diagram when the circulating current suppression method of the present invention is adopted when a circulating current occurs in the power module. In FIG5 and FIG6, the current i of any phase in the input of the power module 10 is used as an example. In FIG7, the voltage VC1 on the first capacitor C1 and the voltage VC2 on the second capacitor C2 are represented by a solid line and a dotted line, respectively. When a circulating current occurs, as shown in FIG5, the waveform of the current i is affected by the circulating current and is offset. In this example, the circulating current flows from the output of the power module 10 into the power module 10, causing the waveform of the current i to offset toward a negative value. In this case, by adopting the circulating current suppression method of the present invention, as shown in Figures 6 and 7, operating the first switch S1 and / or the second switch S2 to generate an inductive current flowing from the second end of the energy storage inductor L to the first end, so as to charge the first capacitor C1 and increase the voltage VC1 on the first capacitor C1, thereby suppressing the current offset caused by the circulating current and returning the waveform of the current i to the state when there is no circulating current.

[0055] In summary, this case provides an electric power system and a method for suppressing circulating current therewith, wherein the electric power system has multiple power modules. When circulating current occurs in any power module, this case operates the balancing circuit in the power module to adjust the neutral voltage of the balancing circuit to suppress the circulating current.

[0056] It should be noted that the above description is merely a preferred embodiment for the purpose of illustrating the present invention. The present invention is not limited to the described embodiment. The scope of the present invention is determined by the appended claims. Furthermore, the present invention is subject to various modifications as conceived by those skilled in the art, without departing from the intended scope of the appended claims.

Claims

1. A circulating current suppression method is applied to a power system having a plurality of power modules, and each of the power modules comprises a high voltage bus, a low voltage bus and a balancing circuit having a neutral voltage, the circulating current suppression method comprising: In each of the balancing circuits, a first capacitor is provided to electrically couple between the high voltage bus and the neutral voltage, and a second capacitor is provided to electrically couple between the neutral voltage and the low voltage bus; Obtaining the effective value of the input current of each of the power modules; If it is detected that the effective current value of at least one of the plurality of power modules is not maintained at the current reference value, it is determined that a circulating current occurs in the at least one of the plurality of power modules; as well as The balancing circuit of at least one of the plurality of power modules is operated to charge the first capacitor or the second capacitor to adjust the neutral voltage to suppress the circulating current.

2. The circulation suppression method according to claim 1, further comprising: Selecting at least one of the plurality of power modules and marking it as an abnormal power module; Obtaining a difference between the effective current value of the abnormal power module and the current reference value, and obtaining a compensation voltage according to the difference; In the abnormal power module, obtaining a capacitance voltage difference between the second capacitor and the first capacitor; as well as By comparing the compensation voltage and the capacitor voltage difference, it is determined that the circulating current flows into the abnormal power module from the input of the abnormal power module, or that the circulating current flows into the abnormal power module from the output of the abnormal power module.

3. The circulation suppression method according to claim 2, further comprising: If the compensation voltage is greater than the capacitor voltage difference, it is determined that the circulating current flows into the abnormal power module from the input of the abnormal power module; as well as If the compensation voltage is less than the capacitor voltage difference, it is determined that the circulating current flows from the output of the abnormal power module into the abnormal power module.

4. The circulation suppression method according to claim 2, further comprising: In the balancing circuit, an energy storage inductor, a first switch and a second switch are provided, wherein the first end of the energy storage inductor is electrically coupled to the neutral voltage, the first switch is electrically coupled between the high voltage bus and the second end of the energy storage inductor, and the second switch is electrically coupled between the second end of the energy storage inductor and the low voltage bus; as well as In each of the balancing circuits, the first capacitor or the second capacitor is charged by operating the first switch and / or the second switch.

5. The circulation suppression method according to claim 4, further comprising: If the compensation voltage is greater than the capacitor voltage difference, charging the second capacitor by operating the first switch and / or the second switch; When the second capacitor is charged, the inductor current of the energy storage inductor flows from the second end of the energy storage inductor to the first end of the energy storage inductor.

6. The method for suppressing circulating current according to claim 4, further comprising: If the compensation voltage is less than the capacitor voltage difference, charging the first capacitor by operating the first switch and / or the second switch; When the first capacitor is charged, the inductor current of the energy storage inductor flows from the first end of the energy storage inductor to the second end of the energy storage inductor.

7. The circulation suppression method according to claim 1, further comprising: After the first capacitor or the second capacitor is charged, detecting whether the effective current value of the at least one of the plurality of power modules is maintained at the current reference value; If the effective current value of the at least one of the plurality of power modules is maintained at the current reference value, the operation of the balancing circuit is stopped. 8 . The method for suppressing circulating current according to claim 1 , wherein when it is determined that no circulating current occurs in each of the power modules, the low voltage bus of each of the power modules is maintained at the same reference voltage. 9 . The circulating current suppression method according to claim 1 , wherein the neutral voltage is adjusted to increase the voltage of the low voltage bus by charging the first capacitor or the second capacitor.

10. The circulation suppression method according to claim 1, further comprising: The input of each power module is electrically coupled to an AC input source, and the output of each power module is electrically coupled to a load.

11. The circulating current suppression method according to claim 1, wherein the effective value of the current is the current average value or the current root mean square value.

12. An electric power system having a plurality of power modules, wherein an input of each of the power modules receives the same AC input source, and each of the power modules comprises: A DC power bus, including a high voltage bus and a low voltage bus; Balanced circuit, including: A first capacitor and a second capacitor, wherein the negative terminal of the first capacitor is connected to the positive terminal of the second capacitor, the positive terminal of the first capacitor is electrically connected to the high voltage bus, and the negative terminal of the second capacitor is electrically connected to the low voltage bus; a first switch and a second switch, wherein a first end of the first switch is electrically connected to the high voltage bus, a second end of the first switch is electrically connected to a first end of the second switch, and a second end of the second switch is electrically connected to the low voltage bus; and an energy storage inductor, wherein a first end of the energy storage inductor is connected to the negative terminal of the first capacitor and the positive terminal of the second capacitor, and a second end of the energy storage inductor is connected to the second end of the first switch and the first end of the second switch; and A controller, obtaining an effective value of the input current of each of the power modules; If the controller detects that the effective current value of at least one of the multiple power modules is greater than or less than a current reference value, it is determined that a circulating current occurs in the at least one of the multiple power modules; The controller operates the first switch and / or the second switch of the balancing circuit of at least one of the multiple power modules to allow the energy storage inductor to generate an inductor current, and the current direction of the inductor current is opposite to the circulating current.

13. The power system according to claim 12, wherein the controller is configured to: Selecting at least one of the plurality of power modules and marking it as an abnormal power module; Obtaining a difference between the effective current value of the abnormal power module and the current reference value, and obtaining a compensation voltage according to the difference; In the abnormal power module, obtaining a capacitor voltage difference between the second capacitor and the first capacitor; and By comparing the compensation voltage and the capacitor voltage difference, it is determined that the circulating current flows into the abnormal power module from the input of the abnormal power module, or that the circulating current flows into the abnormal power module from the output of the abnormal power module.

14. The power system according to claim 13, wherein the controller is further configured to: When the compensation voltage is greater than the capacitor voltage difference, determining that the circulating current flows from the input of the abnormal power module into the abnormal power module; and When the compensation voltage is less than the capacitor voltage difference, it is determined that the circulating current flows from the output of the abnormal power module into the abnormal power module.

15. The power system according to claim 13, wherein the controller is further configured to: When the compensation voltage is greater than the capacitor voltage difference, the inductor current of the energy storage inductor flows from the second end of the energy storage inductor to the first end of the energy storage inductor by operating the first switch and / or the second switch; and When the compensation voltage is less than the capacitor voltage difference, the first switch and / or the second switch are operated to allow the inductor current of the energy storage inductor to flow from the first end of the energy storage inductor to the second end of the energy storage inductor.

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