Single phase controlling method and three phase inverting device using the same

A three phase inverting device includes a three phase inverter module and a three phase filter module. The three phase inverter module includes a plurality of switches, each two switches are connected for forming a bridge arm, an input end of each of the bridge arm are coupled for forming a DC end, the DC end is connected to a DC load. The three phase filter module is connected to the three phase inverter module, wherein the three phase filter module includes a plurality of inductances and a plurality of capacitances, the inductances are connected at one side of the capacitances, a portion of the capacitances are connected to a output end of each of the bridge arm of the three phase inverter module, a portion of the inductances are connected to an AC end.

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number 108100101, filed Jan. 2, 2019, which is herein incorporated by reference.

BACKGROUND

Technical Field

The present disclosure relates to a controlling method and an inverting device using the same. More particularly, the present disclosure relates to a single phase controlling method and a three phase inverting device using the same.

Description of Related Art

The demand on the renewable energy is increasing owing to the issue of the global warming becomes more serious. An inverter parallel system has become a mainstream owing to the continuously developed technologies of the renewable energy (e.g. a solar power generation system). The power of the inverter parallel system can be enhanced by connecting a plurality of inverter modules in parallel and is increased with the quantity of the inverter modules. Furthermore, when one of the inverter modules is failed, the other inverter modules can be used as a substitute, thereby achieving a high reliability of the system. The requirement on the voltage and current resistance is also lower in this kind of inverter parallel system. When comparing a three phase inverter with three single phase inverters, the three phase inverter has a constant instantaneous power, and a low voltage ripple can be obtained using a low capacitance value. Furthermore, smaller quantity of the switch can be used, thereby reducing power loss and the circuit manufacturing cost.

It is important to increase a current sharing in a single phase and reduce circulating currents between each phase of the inverter parallel system. The common methods for achieving the current sharing include a centralized controlling method, a client-server controlling method, a circular track controlling method, a distributed logic controlling method and a wireless automatic controlling method, etc. The common methods for reducing the circulating currents include a hardware reducing method, a synchronous controlling method and a switch modulating method, etc. However, since the three phases are coupled with each other, the methods for controlling the current sharing and the circulating currents are complicated, thus leading to resource consumption. Therefore, there is a need to develop a method that can effectively control the current-sharing and the circulating currents.

SUMMARY

According to one aspect of the present disclosure, a single phase controlling method is provided. The single phase controlling method is applied to a three phase inverting device, the three phase inverting device includes a three phase switch, the single phase controlling method includes: inputting a DC current into a DC end of the three phase inverting device; performing a de-coupling procedure to calculate a switching duty ratio using an average value of a voltage between the DC end and a ground of an AC end of the three phase inverting device and generating an electric output of a single phase circuit in accordance with the average value of the voltage and the switching duty ratio; performing a dividing procedure to divide the electric output of the single phase circuit into two current variations of an inverting end of the three phase inverting device, wherein the two current variations are corresponded to an excitation state and a demagnetization state respectively; and performing an integrating procedure to integrate the two current variations corresponded to the excitation state and the demagnetization state for obtaining another switching duty ratio on a next duty of the three phase switch.

According to another aspect of the present disclosure, a three phase inverting device is provided. The three phase inverting device includes a three phase inverter module and a three phase filter module. The three phase inverter module includes a plurality of switches, each two of the switches are connected for forming a bridge arm, an input end of each of the bridge arm are coupled for forming a DC end, the DC end is connected to a DC load. The three phase filter module is connected to the three phase inverter module, wherein the three phase filter module includes a plurality of inductances and a plurality of capacitances, the inductances are connected at one side of the capacitances, a portion of the capacitances are connected to a output end of each of the bridge arm of the three phase inverter module, a portion of the inductances are connected to an AC end.

DETAILED DESCRIPTION

FIG. 1is a flow chart showing a single phase controlling method according to one embodiment of the present disclosure. The single phase controlling method of the present disclosure is applied in a three phase inverting device. The three phase inverting device includes a three phase switch. The single phase controlling method of the present disclosure includes: a step S101for inputting a DC current into a DC end of the three phase inverting device; a step S102for performing a de-coupling procedure to calculate a switching duty ratio using an average value of a voltage between the DC end and a ground of an AC end of the three phase inverting device and generating an electric output of a single phase circuit in accordance with the average value of the voltage and the switching duty ratio; a step S103for performing a dividing procedure to divide the electric output of the single phase circuit into two current variations of an inverting end of the three phase inverting device, wherein the two current variations are corresponded to an excitation state and a demagnetization state respectively; a step S104for performing an integrating procedure to integrate the two current variations corresponded to the excitation state and the demagnetization state for obtaining another switching duty ratio on a next duty of the three phase switch. In the aforementioned single phase controlling method, the de-coupling procedure, the dividing procedure and the integrating procedure can be performed through a controller. Furthermore, a converter parallel system can be obtained through connecting a plurality of the three phase inverting devices in parallel. The operation mechanism of the single phase controlling method of the present disclosure is then described.

For clearly understanding the present disclosure, the following table lists the definitions of the symbols of the circuit of the present disclosure.

symboldefinitionVnOa voltage between a virtual ground n and aground of an inverteruROa voltage of R phase relative to a ground 0of an inverter between switches on the upperarm and the lower armuSOa voltage of S phase relative to a ground 0of an inverter between switches on the upperarm and the lower armuTOa voltage of T phase relative to a ground 0of an inverter between switches on the upperarm and the lower armΔiiRa current variation of R phase at an inverterend of the inductance LiΔiiSa current variation of S phase at an inverterend of the inductance LiΔiiTa current variation of T phase at an inverterend of the inductance LiΔiCRa current variation of a capacitance of R phaseof a LCL filterΔiCSa current variation of a capacitance of S phaseof a LCL filterΔiCTa current variation of a capacitance of T phaseof a LCL filterΔiCgRa current variation of a capacitance of R phaseof a LCL filterΔiCgSa current variation of a capacitance of S phaseof a LCL filterΔiCgTa current variation of a capacitance of T phaseof a LCL filterLiRan inductance Livalue of R phase at a switchend of a LCL filterLiSan inductance Livalue of S phase at a switchend of a LCL filterLiTan inductance Livalue of T phase at a switchend of a LCL filterLgRan inductance Lgvalue of R phase at an AC endof a LCL filterLgSan inductance Lgvalue of S phase at an AC endof a LCL filterLgTan inductance Lgvalue of T phase at an AC endof a LCL filterTa time of a cycleVRpna voltage of R phase relative to a virtual groundn in an AC endVSpna voltage of S phase relative to a virtual groundn in an AC endVTpna voltage of T phase relative to a virtual groundn in an AC endΔiCga current variation of a capacitance of a LCLfilteriCg(t)a current of a capacitance of a LCL filter at aninstant cycleiCg(t − TS)a current of a capacitance of a LCL filter atprevious cycleCga capacitance of a LCL filter LCLVcgn(t)a voltage between two sides of the capacitanceof LCL filter at an instant cycleVcgn(t − TS)a voltage between two sides of the capacitanceof LCL filter at previous cycleVcgn(t − 2TS)a voltage between two sides of the capacitanceof LCL filter at a cycle before the previous cycleTS0a time of S0sate during a cycleTS1a time of S1sate during a cycleTS2a time of S2sate during a cycleTS7a time of S7sate during a cycleS0a state showing that the switches of three phasesare non-conductiveS1a state showing that only the switch of R phaseis conductiveS2a state showing that only the switches of R and Sphases are conductiveS7a state showing that three phases are conductiveDRa switching duty ratio of the switch of R phase RDSa switching duty ratio of the switch of S phase SDTa switching duty ratio of the switch of T phase TVnOan average voltage between a virtual ground nand the ground O of the inverterVnOS0a voltage between the virtual ground n and theground 0 of the inverter at S0stateVnOS1a voltage between the virtual ground n and theground 0 of the inverter at S1stateVnOS2a voltage between the virtual ground n and theground 0 of the inverter at S2stateVnOS7a voltage between the virtual ground n and theground 0 of the inverter at S7stateVRcn(t)a voltage of R phase between two sides of thecapacitance of the LCL filter at an instant cycleVRcn(t − TS)a voltage of R phase between two sides of thecapacitance of the LCL filter at an previous cycleVRcn(t − 2TS)a voltage of R phase between two sides of thecapacitance of the LCL filter at a cycle before theprevious cycleVScn(t)a voltage of S phase between two sides of thecapacitance of the LCL filter at an instant cycleVScn(t − TS)a voltage of S phase between two sides of thecapacitance of the LCL filter at an previous cycleVScn(t − 2TS)a voltage of S phase between two sides of thecapacitance of the LCL filter at a cycle before theprevious cycleVTcn(t)a voltage of T phase between two sides of thecapacitance of the LCL filter at an instant cycleVTcn(t − TS)a voltage of T phase between two sides ofthe capacitance of the LCL filter at a previous cycleVTcn(t − 2TS)a voltage of T phase between two sides of thecapacitance of the LCL filter at a cycle before theprevious cycleDRSTa switching duty ratio of the three phase switchLiRSTa capacitance Livalue at the AC end of the threephase LCL filterLgRSTan inductance Lgvalue at the AC end of the threephase LCL filterΔiiRSTa current variation of the inductance Liat the inverterend of the three phase LCL filterVRSTpna voltage relative to the virtual ground n at the ACendVRSTcN(t)a voltage of the capacitance relative to the groundN of the AC end of the three phase LCL filter atan instant cycleVRSTcN(t − TS)a voltage of the capacitance relative to the groundN of the AC end of the three phase LCL filter at aprevious cycleVRSTcN(t − 2TS)a voltage of the capacitance relative to the groundN of the AC end of the three phase LCL filter at acycle before the previous cycleCgRSTa capacitance value of the three phases of the LCLfilterLgRSTan inductance Lgvalue at the AC end of the threephase LCL filterXan extra term of the duty ratio of the R phaseswitch RYan extra term of the duty ratio of the S phaseswitch SZan extra term of the duty ratio of the T phaseswitch TLikan inductance Livalue at the AC end of any phaseof the three phase LCL filterΔiik,maga current variation of the inductance Liat theexcitation sate at the inverter end of the LCL filterdta time variationLgkan inductance Lgvalue at the AC end of any phaseof the three phase LCL filterΔiCk,maga current variation at the excitation state of any phaseof the three phase LCL filterVkpna voltage at the AC end relative to the virtual groundn of any phase of the three phasesΔiik,dema current variation of the inductance Liat thedemagnetization state at the inverter endΔiCk,dema current variation at the demagnetization state ofany phase of the three phases of the capacitance ofthe LCL filterΔiika current variation of the inductance Liof any phaseof the three phases at the inverter end of the LCLfilterΔiCka current variation of the capacitance of any phaseof the three phases at the inverter end of the LCLfilterΔiik(n + 1)a current variation of the inductance Liof any phaseof the three phases the LCL filter at an instant cycleigk,ref(n + 1)a reference current of the inductance Lgof the LCLfilter at a next cycleigk,ref(n)a reference current of the inductance Lgof the LCLfilter at an instant cycleii(n)a current of the inductance Liof single phase of theLCL filter at an instant cycleiik(n)a current of the inductance Liof any phase of thethree phases at an instant cyclei*gk,ref(n + 1)a term used for substituting a portion of Δiik(n + 1)Lge(i)an evaluation value of the inductance Lgat the ACend of the LCL filter LCLLie(i)an evaluation value of the inductance Liat the inverterend of the LCL filter LCLLTe(i)a summation of Lge(i) and Lie(i)dGC(n + 1)a duty ration at a next cyclevkpn(n)a voltage relative to the virtual ground n of any phaseof the three phasesvdc(n)a DC voltage of the inverter end at an instant cycleick(n)a current of the capacitance of any phase of the threephases of the LCL filter at an instant cycleick(n − 1)a current of the capacitance of any phase of the threephases of the LCL filter at an previous cyclevkcn(n)a voltage relative to the virtual ground n of thecapacitance of any phase of the three phases of theLCL filter at an instant cyclevkcn(n − 1)a voltage relative to the virtual ground n of thecapacitance of any phase of the three phases of theLCL filter at an previous cyclevkcn(n − 2)a voltage relative to the virtual ground n of thecapacitance of any phase of the three phases ofthe LCL filter at a cycle before the previous cycleVRNa voltage of R phase which relative to a ground Nof a supply mains at the AC endVSNa voltage of S phase which relative to a ground Nof a supply mains at the AC endVTNa voltage of T phase which relative to a ground Nof a supply mains at the AC endLlan equivalent inductance at the AC endSCRa short circuit ratio of the power grid of the supplymainsIIRa current of the equivalent inductance of R phaseat the AC endIISa current of the equivalent inductance of S phaseat the AC endIITa current of the equivalent inductance of T phaseat the AC endIgR1a current of the inductance Lgof R phase of theLCL filter of the first inverter in the inverterparallel systemIgS1a current of the inductance Lgof S phase of theLCL filter of the first inverter in the inverterparallel systemIgT1a current of the inductance Lgof T phase of theLCL filter of the first inverter in the inverterparallel systemIgR2a current of the inductance Lgof R phase of theLCL filter of the second inverter in the inverterparallel systemIgS2a current of the inductance Lgof S phase of theLCL filter of the second inverter in the inverterparallel systemIgT2a current of the inductance Lgof T phase of theLCL filter of the second inverter in the inverterparallel systemIgR3a current of the inductance Lgof R phase of theLCL filter of the third inverter in the inverterparallel systemIgS3a current of the inductance of S phase of theLCL filter of the third inverter in the inverterparallel systemIgT3a current of the inductance Lgof T phase of theLCL filter of the third inverter in the inverterparallel system

FIG. 2is a schematic view showing a circuit of a three phase inverting device using the single phase controlling method ofFIG. 1. InFIG. 2, the three phase inverting device includes a three phase inverter module110and a three phase filter module120. In the embodiment, the switches S1-S6of the three phase inverter module110are arranged to form a full-bridge inverter circuit, however, it is also possible to use a half-bridge inverter circuit or other type of the inverter circuit, there is no limitations. The three phase filter module120includes inductances L1-L6and capacitances C1-C3, thereby forming a three phase LCL filter. The three phase inverter module110includes a DC end DCT, and three phases are represented by symbols R, S ant T respectively. The switch S1and the switch S2are connected to form a bridge arm B1; the switch S3and the switch S4are connected to form a bridge arm B2; and the switch S5and the switch S6are connected to form a bridge arm B3. An input end of each of the bridge arms B1-B3are connected as the DC end DCT, which connects to a front end of a DC load (e.g. solar cell) for receiving or outputting a DC voltage VDC. An output end of each of the bridge arms B1-B3is connected to one end of each of the inductances L1, L3and L5. The other end of each of the inductances L1, L3and L5is connected to each of the capacitances C1-C3. One end of each of the inductances L2, L4and L6is connected to each of the capacitances C1-C3. The other end of each of the inductances L2, L4and L6is connected to a three phase power of an AC end ACT (e.g. supply mains or other AC power source).

Each of the switches S1-S6can be controlled independently by a controlling signal so that a conductive state of each of the switches S1-S6can be controlled. The switches S1, S3and S5are located in an upper arm of the bridge arms B1-B3. The switches S2, S4and S6are located in a lower arm of the bridge arms B1-B3. The switches in the same arm will be conducted with each other alternatively in accordance with the signal received (e.g. the switches S1and S2are conducted with each other alternatively; the switches S3and S4are conducted with each other alternatively; and the switches S5and S6are conducted with each other alternatively). Therefore, voltages uROuSOand uTOare generated on the output end of the bridge arms B1-B3in accordance with the DC voltage VDC. The inductances L1-L6can store or release energy in accordance with voltage variations of the voltages uROuSoand uTO. The capacitances C1-C3have filtering effect. Therefore, an electric power can be transformed between the DC end DCT and the AC end ACT of the three phase inverting device.

The operation mechanism of the single phase controlling method of the present disclosure is then described in the following paragraphs.

Assuming that VnOrepresents a voltage of the three phases which are all connected to a ground n and a ground O, the VnOcan be represented by the following equations (1), (2) and (3) in accordance with a Kirchhoff's law:

The equations (1), (2) and (3) can be combined to equation (4), and ΔiCgcan be replaced by a voltage of a capacitance, which is shown in equation (5):

As shown inFIG. 3, when the three phase switches switching signals, a cycle can be divided into six regions, seven states S0, S1, S2, S3, S4, S5, S6and S7can be presented in each of the regions, and four states SO, S1, S2and S7can be presented in some regions. Therefore, the occupied time T can be shown with a duty ratio of the three phase switches and voltages uRO, uSOand uTO, in the following TABLE 1.

Accordingly,VnOcan be represented by the following equation (6):

Therefore,VnOcan be represented by the following equation (11)

Thus, a switching duty ratio of the three phase switch can be represented by the following equation (12):

An aspect of the present disclosure is to use a single phase to control three phases. Therefore, it is needed to prove that the duty ratio of each of the three phases R, S and T is the same, thereby completing the de-coupling procedure. From the above equation (12), it is known that

Substituting the equations (13), (14) and (15) to the equation (11), an equation (16) can be derived as follows:

Assuming that the terms X, Y and Z are equal,VnOcan be represented by the equation (17):
VnO=VDCX=VDCY=VDCZ(17).

Therefore, the duty ratio of the three phase switch returns back to the equation (12), showing that the three phases can be de-coupled to three single phases. Therefore, an operation of the three phases can be controlled by controlling three independent single phases being de-coupled.

The dividing procedure as shown inFIG. 3, in an excitation state, an equation (18) can be derived in accordance with a Kirchhoff's law, and an equation (19) can be used to represent an excitation current variation Δiik,magof the inverter end, where k represents any one of the three phases R, S and T, the equations (18) and (19) are as follows:

In a demagnetization state, an equation (20) can also be derived in accordance with a Kirchhoff's law, and an equation (21) can be used to represent a demagnetization current variation Δiik,demof the inverter end, the equations (20) and (21) are as follows:

In the integrating procedure, when combining the excitation current Δiik,magin the equation (19) and the demagnetization current Δiik,demin the equation (21), an equation (22) can be obtained as follows:

The current in the converter end in the next cycle can be represented as an equation (23), which can be viewed as a summation of a difference between a reference current in the instant cycle and the reference current in the next cycle and a difference between the reference current in the instant cycle and the current in the converter end, the equation (23) is as follows:
Δiik(n+1)={igk,ref(n+1)−igk,ref(n)}+{igk,ref(n)−ii(n)}=igk,ref(n+1)−iik(n)  (23)

The switching duty ratio in the next cycle can be obtained using the equations (22) and (23), which is represented as an equation (24), in which i*gk,ref(n+1) can be represented by an equation (25), and the equation (25) can be rewritten to an equation (26):

FIG. 5is schematic view showing that the three phase inverting devices ofFIG. 2are connected in parallel for forming an inverter parallel system. In the embodiment, the DC end of each of the three phase inverting device are connected in parallel for receiving and outputting the current voltage VDC, and an output end of the three phase filter module120are connected in parallel, therefore a three phase power can be provided to an AC circuit in the back end, or the three phase power can be received through the AC circuit for inverting. In this kind of client-server architecture, one of the three phase inverting devices is functioned as a server and takes charge of a modulation of an output voltage, the other of the three phase inverting devices is functioned as a client and takes charge of tracing current commands send by the main three phase inverting device. Therefore, a current-sharing control can be achieved, and each of the three phase inverting devices has an equivalent power output. Furthermore, in a conventional inverter parallel system, the value of the output current of each of the inverters is inconsistent, thus unbalanced currents are generated in the system, so called circulating currents. In the inverter parallel system of the present disclosure, the single phase controlling method is applied to each of the three phase inverting devices. The circuit of each phase of the three phase inverting device is treated as an independent single circuit, and the current command of each of the three phase inverting devices is traced independently. Therefore, the circulating current between each of the three phase inverting devices can be dramatically reduced. The effects of the single phase controlling method of the present disclosure applied to the aforementioned inverter parallel system are then described in the following paragraphs.

In an example, the three phase inverting device of the present disclosure is applied in an AC end (e.g., a supply mains). The parameter settings in a simulation circuit are shown in TABLE 2.

TABLE 2DC current inputted from the inverter end400 VAC voltage of the AC end220 Vswitching frequency of the switch20 kHzpower30 kVAinductance Likof the filter1.5 mHcapacitance Cgkof the filter10 μHinductance Lgkof the filter0.5 mHeffective inductance Llkof the transmission line0~146.2 μH

For evaluating the efficiency of the single phase controlling method of the present disclosure, five times of harmonic wave and seven times of harmonic wave are inputted to the system, as shown in TABLE 3. The AC voltage of the AC end is set to an unbalance state, as shown in TABLE 4.

FIG. 6is a schematic view showing a comparison of three phase currents in an AC end and an inverter end when a supply mains is in four different power factors and in a strong power grid. The strong power grid indicates that (L1=0, SCR=∞). The current and the THD (Total Harmonic Distortion) ofFIG. 6are shown in TABLE 5. In TABLE 5, it is shown that the single phase controlling method of the present disclosure is performed under the strong power grid. When the power factor is 1 or −1, the THD is smaller than 0.93%. When the power factor is 0, and the current lags the voltage, the THD is smaller than 0.92%. When the power factor is 0, and the current leads the voltage, the THD is smaller than 0.95%.

FIG. 7is a schematic view showing a comparison of three phase currents in an AC end and an inverter end when a supply mains is in four different power factors and in a weak power grid. The strong power grid indicates that (L1=146.2 μH, SCR=10). The current and the THD (Total Harmonic Distortion) ofFIG. 7are shown in TABLE 6. In TABLE 6, it is shown that the single phase controlling method of the present disclosure is performed under the strong power grid. When the power factor is 1 or −1, the THD is smaller than 0.96%. When the power factor is 0, and the current lags the voltage, the THD is smaller than 0.94%. When the power factor is 0, and the current leads the voltage, the THD is smaller than 0.98%.

In sum, the three phase inverting device of the present disclosure can be controlled using a single phase after performing the de-coupling procedure, and complicated procedures (e.g. a abc to dq step) can be omitted when using the dividing procedure and the integrating procedure. Furthermore, through the evaluation result, it is shown that the current of each of the three phases can be precisely controlled, thereby achieving automatic current-sharing in a single phase, and the circulating current can also be reduced. Therefore, the inverting procedures can be dramatically simplified.