TWI888858B - Power system and circulating current suppression method thereof - Google Patents

Abstract

The present disclosure provides a circulating current suppression method applied in a power system with a plurality of power modules, each including a high voltage bus, a low voltage bus and a balance circuit having a neutral voltage. The circulating current suppression method includes: in each balance circuit, disposing a first capacitor electrically coupled between the high voltage bus and the neutral voltage, and disposing a second capacitor electrically coupled between the neural voltage and the low voltage bus; acquiring an effective current value of the input of each power module; if the effective current value of at least one power module is not maintained at a reference current value, determining that the at least one power module has a circulating current; and operating the balance circuit of the power module which has the circulating current to charge the first or second capacitor for regulating the neutral voltage and suppressing the circulating current.

Description

Power system and circulating current suppression method thereof

This case relates to an electric power system and a circulating current suppression method thereof, and in particular to 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.

In the application of multi-module parallel connection, the size of the module is crucial because of the large number of modules connected in parallel. In order to reduce the size of the module, the front-end AC/DC topology often uses a unidirectional multi-level converter, and the negative pole of the output end of the rear-end DC/DC is directly connected to the negative end of the DC bus. According to the above circuit architecture, when the voltage of the DC bus of each module is different, a DC circulating current will be generated. The circulating current will reduce the power conversion efficiency, and because the front-end AC/DC is a unidirectional topology, the circulating current will cause the input current to generate many harmonics and affect the total harmonic distortion of the input current. In addition, excessive circulating current will cause the neutral point potential of the multi-stage converter to shift and affect the normal operation of the overall system.

Therefore, how to develop an electric power system and a circulating current suppression method thereof that can improve the above-mentioned prior art is an urgent need at present.

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

To achieve the above-mentioned purpose, the present invention provides a method for suppressing circulating current, which is applied to a power system having a plurality of power modules, and each power module includes a high voltage bus, a low voltage bus and a balancing circuit having a neutral voltage. The method for suppressing circulating current includes: in each balancing circuit, a first capacitor is arranged to be electrically coupled between the high voltage bus and the neutral voltage, and a second capacitor is arranged to be electrically coupled between the neutral voltage and the low voltage bus; the effective value of the input current of each power module is obtained; if it is detected that the effective value of the current of at least one power module is not maintained at the current reference value, it is determined that the at least one power module has a circulating current; and the balancing circuit of the power module having the circulating current is operated to charge the first capacitor or the second capacitor to adjust the neutral voltage to suppress the circulating current.

To achieve the above-mentioned purpose, the present case 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 end of the first capacitor is connected to the positive end of the second capacitor, the positive end of the first capacitor is electrically connected to the high voltage bus, and the negative end of the second capacitor is electrically connected to the low voltage bus. The first end of the first switch is electrically connected to the high voltage bus, the second end of the first switch is connected to the first end of the second switch, and the second end of the second switch is electrically connected to the low voltage bus. The first end of the energy storage inductor is connected to the negative end of the first capacitor and the positive end of the second capacitor, and the 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. 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 the current reference value, it is determined 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 inductor current, and the current direction of the inductor current is opposite to the circulating current.

Some typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various variations in different aspects without departing from the scope of the present invention, and the descriptions and illustrations therein are essentially for illustrative purposes rather than for limiting the present invention.

FIG. 1 is a schematic diagram of the circuit architecture of an electric power system of an embodiment of the present invention. As shown in FIG. 1, the electric power system 1 includes a plurality of power modules 10, wherein the number of the power modules 10 is an integer greater than or equal to 2, wherein the input of each power module 10 receives the same AC input source 2, and the output of each power module 10 is electrically coupled to the same load. Generally, the circulating current occurs when a plurality of power modules 10 are connected in parallel, but the present invention is not limited thereto.

In the power system 1, each power module 10 includes a DC power bus, a balancing circuit 11, and a controller 12. The AC input source 2 in this embodiment provides three-phase AC power, but is not limited thereto. In another embodiment, the AC input source 2 can also provide single-phase AC power.

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 end of the first capacitor C1 is electrically connected to the high voltage bus 13, the negative end of the first capacitor C1 is connected to the positive end of the second capacitor C2, and the negative end 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 regarded as a positive DC bus capacitor and a negative DC bus capacitor, respectively. The first end of the first switch S1 is electrically connected to the high voltage bus 13, the second end of the first switch S1 is connected to the first end of the second switch S2, and the second end of the second switch S2 is electrically connected to the low voltage bus 14. The first end of the energy storage inductor L is connected to the negative end of the first capacitor C1 and the positive end of the second capacitor C2, and the second end of the energy storage inductor L is connected to the second end of the first switch S1 and the first end of the second switch S2. In addition, the balancing circuit 11 has a neutral voltage Vn, wherein the neutral voltage Vn couples the negative end of the first capacitor C1, the positive end of the second capacitor C2 and the first end of the energy storage inductor L.

In FIG. 1 , when the power system 1 operates in an ideal state, the low voltage bus 14 of each power module 10 is maintained at the same reference voltage, and no circulating current occurs. When any of the multiple power modules 10 is abnormal, the low voltage bus 14 will not be maintained at the reference voltage, so that the power system 1 generates a circulating current flowing into the abnormal power module 10. Generally, the voltage of the low voltage bus 14 of the abnormal power module 10 is lower than the reference voltage.

The controller 12 obtains the effective current value iz of the input of each power module 10, wherein the effective current value iz may 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 the current reference value iref, the controller 12 determines that a circulating current occurs in the at least one power module 10. Correspondingly, the controller 12 operates the first switch S1 and the second switch S2 of the balancing circuit 11 of the power module 10 where the circulating current occurs, so that the energy storage inductor L generates an inductor current, so that the first capacitor C1 or the second capacitor C2 is charged, so as to adjust the neutral voltage Vn to offset the influence of the circulating current, thereby suppressing the circulating current, wherein the current direction of the inductor current is opposite to the circulating current.

In some embodiments, each power module 10 further includes a conversion circuit 15, which receives the AC input source 2 and outputs DC power to the high voltage bus 13 and the low voltage bus 14 of the balancing circuit 11. The conversion circuit 15 can be, for example but not limited to, a Vienna converter (as shown in FIG. 2 ), and is used 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, which receives DC power via the high voltage bus 13 and the low voltage bus 14. The DC-DC converter 16 can 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. In addition, the output of each power module 10 is connected to the same load 3.

Please refer to FIG. 1 and FIG. 3. FIG. 3 is a flow chart of a method for suppressing circulating current in an electric power system according to an embodiment of the present invention. This method for suppressing circulating current is applicable to the electric power system 1 shown in FIG. 1 and is executed by the controller 12 of the electric power system 1. As shown in FIG. 3, first, in step ST1, a first capacitor C1 and a second capacitor C2 are set in each balancing circuit 11. Then, 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 the at least one power module 10 has a circulating current. 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 result of step ST3 is yes, that is, the effective current value iz of all power modules 10 is maintained at the current reference value iref, step ST2 is executed again.

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 a circulating current occurs in the power module 10, the neutral voltage Vn is adjusted 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.

Please refer to Figure 1 and Figure 4. Figure 4 illustrates the control strategy of the circulating current suppression method of the power system of this case. It should be noted that the control of the power system 1 mentioned in this case is all executed by the controller 12, and no further description is given hereinafter. 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 current effective value iz and the current reference value iref of the abnormal power module is obtained, and the compensation voltage Vc is obtained according to the difference. At the same time, a capacitance voltage difference Vd between the second capacitor C2 and the first capacitor C1 in the abnormal power module is obtained, wherein the capacitance voltage difference Vd is equal to the voltage on the second capacitor C2 minus the voltage on the first capacitor C1.

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, it is determined that the circulating current flows from the input of the abnormal power module into 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. On the contrary, if the compensation voltage Vc is less than the capacitor voltage difference Vd, it is determined that the circulating current flows from the output of the abnormal power module into the abnormal power module, and the first switch S1 and/or the second switch S2 are operated to charge the first capacitor C1, and 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, the reference value iLref of the inductor current is obtained according to the difference between the compensation voltage Vc and the capacitor voltage difference Vd, and the reference value iLref of the inductor current is compared with the actual value iL, and the control signals of the first switch S1 and the second switch S2 are generated according to the comparison result. Thereby, the charging time of the first capacitor C1 or the second capacitor C2 can be determined based on the difference between the compensation voltage Vc and the capacitor voltage difference Vd, and the first switch S1 and/or the second switch S2 can be operated accordingly.

In addition, after the first capacitor C1 or the second capacitor C2 in the power module 10 generating the circulating current is charged, it is detected again whether the effective current value iz of the power module 10 is maintained at the current reference value iref. If the effective current value iz is maintained at the current reference value iref, the operation of the balancing circuit 11 of the power module 10 is stopped; if the effective current value iz is still not maintained at the current reference value iref, the balancing circuit 11 is operated again to charge the first capacitor C1 or the second capacitor C2.

FIG. 5 is a current waveform simulation diagram when a circulating current is not suppressed when a circulating current occurs in the power module, FIG. 6 is a current waveform simulation diagram when a circulating current suppression method of the present case is adopted when a circulating current occurs in the power module, and FIG. 7 is a capacitor voltage waveform simulation diagram when a circulating current suppression method of the present case is adopted when a circulating current occurs in the power module. In FIG. 5 and FIG. 6, the current i of any phase in the input of the power module 10 is taken as an example. In FIG. 7, 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 FIG. 5 , the waveform of the current i is affected by the circulating current and deviates. 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 deviate toward a negative value. In this case, by adopting the circulating current suppression method of the present case, as shown in FIG. 6 and FIG. 7 , the first switch S1 and/or the second switch S2 is operated to generate an inductor 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 deviation caused by the circulating current and returning the waveform of the current i to the state when there is no circulating current.

In summary, the present invention provides a power system and a method for suppressing circulating current, wherein the power system has a plurality of power modules. When a circulating current occurs in any power module, the present invention operates a balancing circuit in the power module to adjust the neutral voltage of the balancing circuit to suppress the circulating current.

It should be noted that the above is only a preferred embodiment for illustrating the present invention. The present invention is not limited to the above embodiment. The scope of the present invention is determined by the scope of the attached patent application. Moreover, the present invention can be modified in various ways by a person skilled in the art, but it does not deviate from the scope of the attached patent application.

1: Power system 10: Power module 2: AC input source 11: Balance circuit 12: Controller 13: High voltage bus 14: Low voltage bus C1: First capacitor C2: Second capacitor S1: First switch S2: Second switch L: Energy storage inductor Vn: Neutral voltage iz: Effective current value i: Current iref: Current reference value ST1, ST2, ST3, ST4: Steps Vc: Compensation voltage Vd: Capacitor voltage difference 15: Conversion circuit 16: DC-DC converter 3: Load iLref: Reference value of inductor current iL: Actual value of inductor current VC1: Voltage on first capacitor C1 VC2: Voltage on the second capacitor C2

FIG. 1 is a schematic diagram of the circuit structure of the power system of an embodiment of the present invention.

FIG. 2 is a schematic diagram of a circuit structure of a specific implementation of the power system of FIG. 1.

FIG. 3 is a flow chart of a method for suppressing circulating current in an electric power system according to an embodiment of the present invention.

FIG. 4 illustrates the control strategy of the circulating current suppression method of the power system in this case.

Figure 5 is a simulation diagram of the current waveform when a circulating current occurs in the power module without suppressing the circulating current.

FIG. 6 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.

FIG. 7 is a simulation diagram of the capacitor voltage waveform when the circulating current suppression method of this case is adopted when a circulating current occurs in the power module.

ST1, ST2, ST3, ST4: Steps

Claims (15)

A circulating current suppression method is applied to a power system having a plurality of power modules, wherein each of the power modules includes a high voltage bus, a low voltage bus and a balanced circuit having a neutral voltage. The circulating current suppression method includes: in each of the balanced 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. ; obtain a current effective value of an input of each of the power modules; if it is detected that the current effective value of at least one of the multiple power modules is not maintained at a current reference value, determine that a circulating current occurs in at least one of the multiple power modules; and operate the balancing circuit of at least one of the multiple power modules to charge the first capacitor or the second capacitor to adjust the neutral voltage to suppress the circulating current. The method for suppressing the circulating current as described in claim 1 further includes: 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; obtaining a capacitance-voltage difference between the second capacitor and the first capacitor in the abnormal power module; and determining whether the circulating current flows into the abnormal power module from the input of the abnormal power module, or the circulating current flows into the abnormal power module from one of the outputs of the abnormal power module by comparing the compensation voltage and the capacitance-voltage difference. The method for suppressing the circulating current as described in claim 2 further includes: if the compensation voltage is greater than the capacitor voltage difference, it is determined that the circulating current flows from the input of the abnormal power module to the abnormal power module; and 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 to the abnormal power module. The circulating current suppression method as described in claim 2 further includes: in the balancing circuit, an energy storage inductor, a first switch and a second switch are provided, wherein a 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 a 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; and in each of the balancing circuits, by operating the first switch and/or the second switch, the first capacitor or the second capacitor is charged. The circulating current suppression method as described in claim 4 further includes: if the compensation voltage is greater than the capacitor voltage difference, the second capacitor is charged by operating the first switch and/or the second switch; wherein 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. The circulating current suppression method as described in claim 4 further includes: if the compensation voltage is less than the capacitor voltage difference, the first capacitor is charged by operating the first switch and/or the second switch; wherein 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. The circulating current suppression method as described in claim 1 further includes: after the first capacitor or the second capacitor is charged, detecting whether the effective current value of at least one of the multiple power modules is maintained at the current reference value; if the effective current value of at least one of the multiple power modules is maintained at the current reference value, stopping the operation of the balancing circuit. A method for suppressing circulating current as described in 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. A circulating current suppression method as described in 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. The circulating current suppression method as described in claim 1 further includes: setting the input electrical coupling of each power module to an AC input source, and setting the output electrical coupling of each power module to a load. A circulating current suppression method as described in claim 1, wherein the effective value of the current is a current average value or a current root mean square value. A power system with multiple power modules, wherein an input of each power module receives the same AC input source, and each power module includes: a DC power bus, including a high voltage bus and a low voltage bus; a conversion circuit, electrically connected between the AC input source and the DC power bus, configured to receive the AC input source and output DC power to the DC power bus; a balancing circuit , comprising: a first capacitor and a second capacitor, wherein the negative end of the first capacitor is connected to the positive end of the second capacitor, the positive end of the first capacitor is electrically connected to the high voltage bus, and the negative end of the second capacitor is electrically connected to the low voltage bus; a first switch and a second switch, wherein the first end of the first switch is electrically connected to the high voltage bus, and the second end of the first switch is connected to the first end of the second switch, and the second end of the second switch is electrically connected to the low voltage bus; and an energy storage inductor, wherein the first end of the energy storage inductor is connected to the negative end of the first capacitor and the positive end of the second capacitor, and the 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 a current effective value of the input of each power module; If the controller The device 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, and then determines that a circulating current occurs in at least one of the multiple power modules; wherein the controller operates the first switch and/or the second switch of the balancing circuit of at least one of the multiple power modules, so that the energy storage inductor generates an inductor current, and a current direction of the inductor current is opposite to the circulating current. The power system as claimed in claim 12, wherein the controller is used to: select at least one of the multiple power modules and mark it as an abnormal power module; obtain a difference between the effective current value of the abnormal power module and the current reference value, and obtain a compensation voltage according to the difference; obtain a capacitance voltage difference between the second capacitor and the first capacitor in the abnormal power module; and by comparing the compensation voltage and the capacitance voltage difference, determine whether the circulating current flows into the abnormal power module from the input of the abnormal power module, or the circulating current flows into the abnormal power module from one of the outputs of the abnormal power module. The power system as described in claim 13, wherein the controller is further used to: when the compensation voltage is greater than the capacitor voltage difference, determine that the circulating current flows from the input of the abnormal power module to the abnormal power module; and when the compensation voltage is less than the capacitor voltage difference, determine that the circulating current flows from the output of the abnormal power module to the abnormal power module. The power system as claimed in claim 13, wherein the controller is further used to: when the compensation voltage is greater than the capacitor voltage difference, by operating the first switch and/or the second switch, allow the inductor current of the energy storage inductor to flow from the second end of the energy storage inductor to the first end of the energy storage inductor; and when the compensation voltage is less than the capacitor voltage difference, by operating the first switch and/or the second switch, 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.

Images