CN108512433B - Method for realizing accurate phase shift based on multi-module cascade matrix converter - Google Patents

Abstract

The present invention provides a method for realizing precise phase shifting based on a multi-module cascaded matrix converter. The first action time t1 of the internal switch and the second action time t2 of the switch; i=1,2,...,N, N is the number of modules of the converter; S2, if t1 and t2 do not meet the safe commutation rules, according to the safe commutation The rules adjust t1 and t2; the safe commutation rule is used to ensure that the i-th module can achieve safe commutation and has the same duty cycle as other modules in any switching control cycle; S3, repeat S1 and S2, and obtain the i-th module’s duty cycle. t1 and t2 of all switch control periods; S4, repeat S1 to S3, obtain t1 and t2 of all switch control periods of all modules to perform phase shift control on the converter. The method provided by the invention improves the output quality and enhances the operation safety.

Description

A Realization Method of Accurate Phase Shift Based on Multi-module Cascaded Matrix Converter

technical field

The invention relates to the technical field of power electronics, in particular to a method for realizing precise phase shifting based on a multi-module cascaded matrix converter.

Background technique

At present, focusing on energy conservation and emission reduction, effectively reducing the energy consumption of enterprises has become the focus of people's attention. The application of medium/high voltage high-power inverter device greatly improves the power saving rate of each motor, and the energy saving effect is remarkable. Multi-module matrix converters are currently the only matrix converters commercialized for medium and high voltage applications.

Multi-module matrix converters combine traditional power converters with multi-level technology, and have the dual advantages of matrix converters and H-bridge cascaded high-voltage inverters, which can not only connect multiple independent low-voltage power unit modules in series to achieve high-voltage output , the output is sinusoidal, and it can realize bidirectional flow of energy, four-quadrant operation, and no intermediate DC capacitors.

FIG. 1 is a schematic diagram of a topology structure of a multi-module cascaded matrix converter. Each phase of the multi-module matrix converter A, B, and C three phases is formed by cascading a plurality of module matrix converters. In order to obtain more output voltage levels, the expected output voltages of each module are equal, and the switch state adopts a cyclic shift method. Therefore, the phase-shift PWM method is a PWM method specially used for cascaded multilevel converters. There is a certain phase shift between the PWM pulses of each module, that is, the phase is staggered, so that the equivalent switching frequency of the PWM wave output by each module is increased to several times the original, so that the switching frequency can be increased without increasing the switching frequency. conditions, greatly reducing the output harmonics.

Conventional multi-module cascaded matrix converters usually use conventional approximate phase-shifting methods. However, the conventional approximate phase-shifting method only bluntly shifts the phases of the PWM waveforms of each module in sequence, so that the average output voltage of each module cannot be guaranteed to be equal. Output quality cannot be guaranteed.

SUMMARY OF THE INVENTION

In order to solve the problems existing in the prior art, the present invention provides a method for realizing precise phase shifting based on a multi-module cascaded matrix converter.

In one aspect, the present invention proposes a phase-shifting method based on a multi-module cascaded matrix converter, comprising: S1, calculating, according to the phase-shifted switching waveform of the ith module of the multi-module cascaded matrix converter, calculating The first switch operation time t1 and the switch second operation time t2 in any switch control cycle corresponding to the i-th module; i=1,2,...,N, N is the multi-module cascaded matrix converter The number of modules; S2, determine whether t1 and t2 in any switch control cycle corresponding to the i-th module conform to the preset safety commutation rules, if not in line with the safety commutation rules, then according to the safety The commutation rule adjusts t1 and t2; the safe commutation rule is used to ensure that the ith module can realize safe commutation and the duty cycle of the ith module and other modules in any switching control period The same; S3, repeat steps S1 and S2 to obtain t1 and t2 of all switch control cycles of the i-th module; S4, repeat steps S1 to S3, obtain all modules in the multi-module cascaded matrix converter t1 and t2 of all switch control periods, and phase-shift control of the multi-module cascaded matrix converter according to t1 and t2 of all switch control periods of all modules.

Preferably, the safe commutation rules include:

Among them, T is the switching control period, and t is the on-time of the switching device.

t_P为开关器件动作的时间，N_C为控制器在一个开关控制周期T的计数值。Preferably, the first safe commutation time is 11n, the second safe commutation time is 5n, and the minimum pulse width is 2n; wherein,

t _P is the action time of the switching device, and N _C is the count value of the controller in one switching control period T.

Preferably, the step S3 further includes: S31, if 5n≤t1≤11n, set t1 to 11n, and extend t2 backward; if t1≤5n, set t1 to 0, and set t2 forward Push forward; S32, if T-t2<5n, set t2 to T-5n, and push t1 forward.

Preferably, the method further includes: S0, phase shifting the switching waveform corresponding to the ith module of the multi-module cascaded matrix converter by (i-1) T/N; wherein T is the switching control period.

Preferably, the module is a three-module cascaded matrix converter.

In yet another aspect, the present invention provides a phase shifting device based on a multi-module cascaded matrix converter, comprising: at least one processor; and at least one memory communicatively connected to the processor, wherein: the memory stores Program instructions executable by the processor, invoking the program instructions by the processor capable of performing the method as previously described.

In yet another aspect, the present invention provides a non-transitory computer-readable storage medium, the non-transitory computer-readable storage medium storing computer instructions, the computer instructions causing the computer to perform the aforementioned method.

The invention provides a method for realizing accurate phase shifting based on a multi-module cascaded matrix converter. By applying a safe commutation rule to adjust the switching action time, the duty cycle of each module in the same switching control period is guaranteed to be the same, thereby realizing Accurate phase shift is ensured, the output voltage of each module is equal, the output quality is improved, and at the same time, the realization of safe commutation of multi-module cascaded matrix converters is ensured, and the safety of converter operation is enhanced.

Description of drawings

1 is a schematic diagram of the topology of a multi-module cascaded matrix converter;

2 is a schematic flowchart of a phase shifting method based on a multi-module cascaded matrix converter according to a specific embodiment of the present invention;

3 is a schematic diagram of a comparison of phase shifting methods according to a specific embodiment of the present invention;

4 is a schematic diagram of a driving voltage waveform when the phase shift is 0 according to a specific embodiment of the present invention;

5 is a schematic diagram of a driving voltage waveform when the phase shift is T/4 according to a specific embodiment of the present invention;

6 is a schematic diagram of a driving voltage waveform when the phase shift is 2T/4 according to a specific embodiment of the present invention;

7 is a schematic diagram of a driving voltage waveform when the phase shift is 3T/4 according to a specific embodiment of the present invention;

FIG. 8 is a schematic structural diagram of a phase shifting device based on a multi-module cascaded matrix converter according to a specific embodiment of the present invention.

Detailed ways

The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and embodiments. The following examples are intended to illustrate the present invention, but not to limit the scope of the present invention.

The matrix converter is a new type of AC-AC power converter. It can realize the transformation of various parameters (phase number, phase, amplitude, frequency) of alternating current. Compared with traditional converters, it has the following advantages: no intermediate DC energy storage link; four-quadrant operation; excellent input current waveform and output voltage waveform; freely controllable power factor. Matrix converters have become one of the hotspots in the research of power electronics technology, and have broad application prospects.

In order to use low-voltage switching devices to achieve high-voltage output in matrix converters, there are currently two mainstream solutions: one solution is to use a half-bridge inverter structure in which power electronic devices are connected in series, that is, a clamped multi-level inverter. Another method is to use the basic power units to directly connect in series to form a cascaded circuit structure. Among them, the cascaded matrix converter needs to achieve the purpose of greatly reducing the output harmonics without increasing the switching frequency through phase shifting. Since the matrix converter has no intermediate DC capacitors, it has high requirements on the accuracy of the output voltage. The conventional approximate phase-shifting method cannot meet the requirement of equal average output voltages of each module, resulting in large current fluctuations and inability to guarantee the output quality.

In view of the above problems, a specific embodiment of the present invention proposes a phase shifting method based on a multi-module cascaded matrix converter:

2 is a schematic flowchart of a phase shifting method based on a multi-module cascaded matrix converter according to a specific embodiment of the present invention. As shown in FIG. 2, a phase shifting method based on a multi-module cascaded matrix converter, Including: S1, according to the phase-shifted switching waveform corresponding to the i-th module of the multi-module cascaded matrix converter, calculating the first switch action time t1 in any switch control cycle corresponding to the i-th module and switch second action time t2; i=1, 2, . Whether the internal t1 and t2 conform to the preset safe commutation rules, if they do not conform to the safe commutation rules, adjust t1 and t2 according to the safe commutation rules; the safe commutation rules are used to ensure that the first The i modules realize safe commutation and the duty cycle of any switch control cycle of each module is the same; S3, repeating steps S1 and S2 to obtain t1 and t2 of all switch control cycles of the i-th module; S4 , repeating steps S1 to S3 to obtain t1 and t2 of all switch control periods of all modules in the multi-module cascaded matrix converter, and to compare the Multi-module cascaded matrix converter for phase shift control.

Specifically, the multi-module cascaded matrix converter is composed of N modules, and the switch in each module operates twice in one switch control cycle.

First, for the phase-shifted waveform of the i-th module of the multi-module cascaded matrix converter, calculate the first switch action time t1 and the switch second action time in any switch control cycle corresponding to the i-th module Time t2, wherein the first action time t1 of the switch is the first action time of the switch of the i-th module in the any switch control cycle, and the second action time t2 of the switch is the first action time of the switch. The time for the second action of the switches of the i modules in any one of the switch control cycles.

Secondly, according to the preset safety commutation rules, judge the t1 and t2 in any switch control cycle corresponding to the i-th module calculated in the previous step, if the t1 and t2 meet the safety commutation flow rule, keep the t1 and t2; otherwise, apply the safe commutation rule to adjust the t1 and t2, so that the ith module can realize safe commutation, and the ith module The duty cycle is the same as that of other modules in the multi-module cascaded matrix converter in any switching control period.

Next, the above steps are repeated to obtain t1 and t2 of the i-th module in all switching control periods.

Finally, repeat all the above steps, and sequentially acquire t1 and t2 of all switching control periods of all modules in the multi-module cascaded matrix converter according to the method of acquiring t1 and t2 of all switching control periods of the i-th module, and The switching waveforms of all the modules are set by applying t1 and t2 of all switching control periods of the all modules, and the multi-module cascaded matrix converter is phase-shifted through the switching waveforms.

In the specific embodiment of the present invention, by applying the safe commutation rule to adjust the switching action time, it is ensured that the duty ratio of each module in the same switching control period is the same, thereby realizing precise phase shift, ensuring that the output voltage of each module is equal, and improving The output quality is guaranteed, and the realization of the safe commutation of the multi-module cascaded matrix converter is ensured, and the safety of the converter operation is enhanced.

Based on the above specific embodiment, a phase shifting method based on a multi-module cascaded matrix converter, the safe commutation rule includes:

Among them, T is the switching control period, and t is the on-time of the switching device.

Specifically, in the above rules, t1≥the first safe commutation time is used to ensure that the switch can operate safely when it operates for the first time in the switching control cycle.

T-t2≥Second safe commutation time, used to ensure that the switch operates safely for the second time before the end of the switch control cycle.

t≥minimum pulse width is used to ensure that the on-time of the switching device is greater than the minimum pulse width that the controller can output, and that the pulse precision output by the controller can meet the requirements of the switching device.

t2≥t1 is used to ensure that the second action time of the switch is after the first action time of the switch, so as to avoid the distortion of the output of the current module caused by the disorder of the sequence.

The specific embodiments of the present invention provide safe commutation rules for ensuring the realization of the safe commutation of the multi-module cascaded matrix converter, which helps to improve the reliability of the converter's operation.

Based on any of the above specific embodiments, a phase shifting method based on a multi-module cascaded matrix converter, the first safe commutation time is 11n, the second safe commutation time is 5n, and the minimum pulse The width is 2n.

Specifically, _NC is the count value of the controller in one switching control period T, and _NC is determined by the frequency of the crystal oscillator used by the controller and the actual control frequency. t _P is the action time of the switching device, which is obtained from the parameters of the switching device selected by the converter. For example, t _P is the maximum value of the on-delay time and the off-delay time of the switching device, but not limited to this. Calculate n from the following formula, where n represents the count value of the controller during the action time of the switching device:

The application n corresponds to the parameters in the safe commutation rule: the first safe commutation time is 11n, the second safe commutation time is 5n, and the minimum pulse width is 2n.

In the specific embodiment of the present invention, the parameter of the safe commutation rule is converted into the count value of the controller, which provides convenience for the subsequent adjustment of t1 and t2.

Based on any of the above specific embodiments, a phase shifting method based on a multi-module cascaded matrix converter, the step S3 further includes:

S31, if 5n≤t1≤11n, set t1 to 11n, and extend t2 backward; if t1≤5n, set t1 to 0, and push t2 forward;

S32, if T-t2<5n, set t2 to T-5n, and push t1 forward.

Specifically, according to the preset safety commutation rules, t1 and t2 in any switch control period corresponding to the i-th module calculated in the previous step are judged:

First, judge whether t1 satisfies t1≥11n, if t1≥11n, then t1 complies with the safe commutation rule, and there is no need to adjust t1;

If t1<11n, adjust t1: further, if 5n≤t1≤11n, then set t1 to 11n and extend t2 backward; otherwise, set t1 to 0 and extend t2 forward push.

Secondly, judge whether t2 satisfies T-t2≥5n, if T-t2≥5n, then t2 complies with the safe commutation rule, and there is no need to adjust t2;

Otherwise, adjust t2, set t2 to T-5n, and push t1 forward.

In the specific embodiment of the present invention, the switching action time is adjusted by the safe commutation rule, which ensures that the output voltages of each module are equal, improves the output quality, and improves the reliability of the converter operation.

Based on any of the above specific embodiments, a phase shifting method based on a multi-module cascaded matrix converter, further comprising: S0, shifting the switching waveform corresponding to the ith module of the multi-module cascaded matrix converter Phase (i-1) T/N; where T is the switching control period.

Specifically, for the phase-shifted waveform of the i-th module of the multi-module cascaded matrix converter, calculate the first switch action time t1 and the switch first action time t1 in any switch control cycle corresponding to the i-th module. Before the second action time t2, the i-th module needs to be phase-shifted, and the phase-shift size is related to the total number of modules of the multi-module cascaded matrix converter and the serial number of the i-th module. The magnitude of the phase shift corresponding to the module is equal to (i-1)T/N, where T is the switching control period and N is the total number of modules.

For example, assuming T=200 and N=4, the first module corresponds to a phase shift of 0, the second module corresponds to a phase shift of 50, the third module corresponds to a phase shift of 100, and the fourth module corresponds to a phase shift of 150.

In the specific embodiment of the present invention, the use of the conventional approximate phase-shifting method lays a foundation for the subsequent precise phase-shifting.

Based on any of the above specific embodiments, a phase shifting method based on a multi-module cascaded matrix converter, wherein the modules are three-module cascaded matrix converters.

Specifically, the multi-module cascaded matrix converter is formed by cascading a plurality of three-module cascaded matrix converters, but in the specific embodiment of the present invention, the modules constituting the multi-module cascaded matrix converter are not limited to this.

In order to better understand and apply a phase shifting method based on a multi-module cascaded matrix converter proposed by the present invention, the present invention performs the following examples, and the present invention is not limited to the following examples.

Example one:

FIG. 3 is a schematic diagram showing the comparison of phase shifting methods according to a specific embodiment of the present invention. As shown in FIG. 3 , FIG. 3( a ) is the basic waveform before phase shifting, FIG. 3( b ) is the current conventional approximate phase shifting method, and FIG. 3 (c) is the phase-shifting method proposed by the present invention. Both Figures 3(b) and 3(c) phase-shift the fundamental waveform in Figure 3(a) by T/4.

During system operation, the duty cycle of the controller (ie, the on-time of the switch) will change to meet the output voltage requirements. But in the same cycle, the duty cycle of each module is the same, so that each module outputs the same expected voltage value. Assuming a period of time T=200, n is calculated to be approximately 1.2053. The switch-on time of the first switch control cycle is 2t=80, the switch-on time of the second switch-control cycle is 2t=100, and the switch-on time of the third switch-control cycle is 2t=120.

It can be seen from Figure 3(b) that when the conventional approximate phase-shifting method is applied for phase-shifting, the switch-on time in the second switching control cycle is only 90, which does not reach the expected 100 in the second switching control cycle. It can be seen that it is difficult to ensure that the output voltages of each module are equal and equal to the expected output voltage by applying the conventional approximate phase-shifting method.

Correspondingly, it can be seen from FIG. 3( c ) that the above-mentioned problem is solved by applying the phase shifting method proposed by the present invention, and it is ensured that the voltage output by each module can be the same as the expected voltage value to meet the power supply requirements.

Example two:

The switching device used in this example is IGBT, the switching action time of IGBT is t _p = 1.5us, the switching control period is T = 250us, the count value of the controller for one switching control period is N _c = 18750, and n = 113 can be obtained from the above calculation (5n=565, 11n=1243).

FIG. 4 is a schematic diagram of a driving voltage waveform when the phase shift is 0 according to a specific embodiment of the present invention. As can be seen from the figure,

t1=t, 565≤t1≤8923

t2=18750-t, 9827≤t2≤18185

In addition, in Fig. 4(a), 0<t<T/4; in Fig. 4(b), T/4<t<T/2.

FIG. 5 is a schematic diagram of the driving voltage waveform when the phase shift is T/4 according to a specific embodiment of the present invention. Referring to FIG. 5, the above-mentioned phase shifting method is applied, and FIG. 5(a) can be divided into two situations for discussion:

时(565≤t≤3445)①

Hour (565≤t≤3445)

时(3445≤t≤4123)②

Hour (3445≤t≤4123)

t1=11n, t1=1243

t2=2t+11n, 8133≤t2≤9489

Similarly, for Figure 5(b), two cases are discussed:

时(4123≤t≤5253)③

Hour (4123≤t≤5253)

t1=2t, 8246≤t1≤10506

t2=0 (t2 is shifted to the end of the cycle), t2=0

时(5253≤t≤8923)④

Hour (5253≤t≤8923)

FIG. 6 is a schematic diagram of the driving voltage waveform when the phase shift is 2T/4 according to a specific embodiment of the present invention. Referring to FIG. 6, applying the above-mentioned phase shifting method, FIG. 6(a) can be divided into two situations for discussion:

时(565≤t≤8132)①

Hour (565≤t≤8132)

时(8132≤t≤8810)②

Hour (8132≤t≤8810)

t1=11n, t1=1243

t2=2t+11n, 17507≤t2≤18185

In the same way, discuss the situation in Figure 6(b):

时(8810≤t≤9940)，不合理③

time (8810≤t≤9940), unreasonable

FIG. 7 is a schematic diagram of the driving voltage waveform when the phase shift is 3T/4 according to a specific embodiment of the present invention. Referring to FIG. 7 , by applying the above-mentioned phase shifting method, FIG. 7( a ) can be divided into two cases for discussion:

时(565≤t≤4123)①

Hour (565≤t≤4123)

时(4123≤t≤5253)②

Hour (4123≤t≤5253)

t1=t2=T-2t, 8244≤t1=t2≤10504

t1=0 (t1 is just at the beginning of the cycle)

In the same way, discuss the situation in Figure 7(b):

时(5253≤t≤5931)①

Hour (5253≤t≤5931)

t1=11n, t1=1243

t2=T-2t+11n, 8131≤t2≤9487

时(5931≤t≤8923)②

Hour (5931≤t≤8923)

FIG. 8 is a schematic structural diagram of a phase shifting device based on a multi-module cascaded matrix converter according to a specific embodiment of the present invention. As shown in FIG. 8 , the device includes: at least one processor 801; 801 at least one memory 802 communicatively connected, wherein: the memory 802 stores program instructions that can be executed by the processor 801, and the processor 801 invokes the program instructions to be able to execute the multi-based multiplexing provided by the above embodiments. A method for phase-shifting a module cascaded matrix converter, for example, comprising: S1, according to the phase-shifted switching waveform of the i-th module of the multi-module cascaded matrix converter, calculating the corresponding to the i-th module The first action time t1 of the switch and the second action time t2 of the switch in any switch control period; i=1,2,...,N, N is the number of modules of the multi-module cascaded matrix converter; S2, judge the Whether t1 and t2 in any switch control cycle corresponding to the i-th module conform to the preset safe commutation rule, if not, then adjust t1 and t2 according to the safe commutation rule; The safe commutation rule is used to ensure that the ith module can realize safe commutation and the duty ratio of the ith module and other modules is the same in any switch control period; S3, repeating steps S1 and S2, acquiring t1 and t2 of all switching control periods of the i-th module; S4, repeating steps S1 to S3, acquiring t1 and t2 of all switching control periods of all modules in the multi-module cascaded matrix converter , and phase-shift control is performed on the multi-module cascaded matrix converter according to t1 and t2 of all switching control periods of all modules.

Embodiments of the present invention further provide a non-transitory computer-readable storage medium, where the non-transitory computer-readable storage medium stores computer instructions, and the computer instructions cause the computer to execute the multi-module cascade-based matrix transformation provided by the corresponding embodiment The phase-shifting method of the converter, for example, includes: S1, according to the switching waveform of the i-th module of the multi-module cascaded matrix converter after phase-shifting, calculating the switching in any switching control period corresponding to the i-th module. The first action time t1 and the second action time t2 of the switch; i=1,2,...,N, N is the number of modules of the multi-module cascaded matrix converter; S2, determine the corresponding module Whether t1 and t2 in any switch control period conform to the preset safe commutation rules, if they do not conform to the safe commutation rules, adjust t1 and t2 according to the safe commutation rules; the safe commutation rules use In order to ensure that the ith module can realize safe commutation and the ith module and other modules have the same duty cycle in any switching control cycle; S3, repeating steps S1 and S2 to obtain the ith module t1 and t2 of all switch control periods of each module; S4, repeat steps S1 to S3 to obtain t1 and t2 of all switch control periods of all modules in the multi-module cascaded matrix converter, and according to all modules Phase-shift control of the multi-module cascaded matrix converter is performed at t1 and t2 of the entire switching control period.

Those of ordinary skill in the art can understand that all or part of the steps of implementing the above method embodiments can be completed by program instructions related to hardware, the aforementioned program can be stored in a computer-readable storage medium, and when the program is executed, execute It includes the steps of the above method embodiments; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk and other media that can store program codes.

Finally, the method of the present application is only a preferred embodiment, and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and rules of the present invention shall be included within the protection scope of the present invention.

Claims (6)

1. A phase shifting method based on a multi-module cascade type matrix converter is characterized by comprising the following steps:

s1, calculating a first switch action time t1 and a second switch action time t2 in any switch control period corresponding to the ith module according to the switch waveform of the ith module of the multi-module cascade matrix converter after phase shifting; i is 1,2, …, and N is the number of modules of the multi-module cascade type matrix converter;

s2, if t1 and t2 in any switch control period corresponding to the ith module do not accord with a preset safe commutation rule, adjusting t1 and t2 according to the safe commutation rule; the safety commutation rule is used for ensuring that the ith module can realize safety commutation and the duty ratio of the ith module and other modules in any one switch control period is the same;

s3, repeating the steps S1 and S2, and obtaining t1 and t2 of all switch control periods of the ith module;

s4, repeating the steps S1 to S3, obtaining t1 and t2 of all switch control periods of all modules in the multi-module cascaded matrix converter, and performing phase-shifting control on the multi-module cascaded matrix converter according to t1 and t2 of all switch control periods of all modules;

wherein the safe commutation rule comprises:

wherein T is a switch control period, and T is the conduction time of the switch device;

the first safe commutation time is 11n, the second safe commutation time is 5n, and the minimum pulse width is 2 n;

wherein,

t_Pfor the time of the switching device action, N_CThe count value of the controller in one switching control period T is counted.

2. The method according to claim 1, wherein the step S3 further comprises:

s31, if t1 is not less than 5n and not more than 11n, setting t1 as 11n and extending t2 backwards; if t1 is less than or equal to 5n, setting t1 to 0 and pushing t2 forward;

s32, if T-T2<5n, set T2 to T-5n and push T1 forward.

3. The method of claim 1, further comprising:

s0, shifting the phase of the switching waveform corresponding to the ith module of the multi-module cascade type matrix converter by (i-1) T/N; wherein, T is a switch control period.

4. The method of claim 1, wherein the module is a three-module cascaded matrix converter.

5. A phase shift apparatus based on a multi-module cascade type matrix converter, comprising:

at least one processor; and at least one memory communicatively coupled to the processor, wherein: the memory stores program instructions executable by the processor, the processor invoking the program instructions to perform the method of any of claims 1 to 4.

6. A non-transitory computer-readable storage medium storing computer instructions that cause a computer to perform the method of any one of claims 1 to 4.

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