JP4362565B2 - Pulse pattern generation method for three-phase current source converter circuit. - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a pulse pattern generation method for a three-phase current source converter circuit.
[0002]
[Prior art]
In the three-phase current source converter circuit as shown in FIG. 1, a pulse width modulation (PWM) is used for various advantages. In the conventional PWM pulse pattern generation method, narrow ON and OFF pulses are generated in the operation region where the DC output voltage is low, and therefore, the minimum switching time of the switching element guaranteed by the interlock circuit is limited. For this reason, the “pulse distribution method of a three-phase PWM current source converter” by Yasuhiko Neba and Kenichi Seto (The Institute of Electrical Engineers of Japan, Electrotechnical D, Vol. 117, No. 10, 1997) is narrow in the low DC output voltage range. In order not to generate a width pulse, a method using a two-phase triangular wave as a carrier wave and two sine waves as modulation waves has been proposed for a three-phase circuit. A similar method using a two-phase triangular wave has been proposed for a single-phase current source converter circuit.
[0003]
By the way, the PWM pulse pattern is determined by the power supply voltage phase, the three-phase sine wave conduction ratio command value (when a conventional sine wave is used), and the control cycle. The three-phase alternating current is usually sufficient to represent a positive / negative relationship in consideration of a power supply voltage cycle of π / 3. In the following, a-phase voltage Va> b-phase voltage Vb> 0, c-phase voltage Vc = − ( Va + Vb) <0/6 period (hereinafter referred to as “first phase period”), and subsequent b-phase voltage Vb> a-phase voltage Va> 0, c-phase voltage Vc = − (Va + Vb) <0 A period of / 6 (hereinafter referred to as “second phase period”) will be described as an example. Here, for simplicity, it is assumed that the conduction ratio command value is in phase with the power supply voltage so as to realize a power factor of 1. That is, the information necessary for generating the PWM pulse is only the power supply voltage phase, the three-phase sine wave conduction ratio command value (Da, Db, Dc), and the control period T.
A constraint condition for pulse generation in the PWM current source converter is that one of the switching elements on the positive side (S1, S2, S3) and the negative side (S4, S5, S6) must always be on. Therefore, the on / off mode can be expressed by a combination of two element numbers that are conducted.
[0004]
FIG. 4 shows the on-off mode transition sequence circulating in the control period in the first and second phase periods of the proposed pulse pattern generation method using a two-phase triangular wave as a carrier wave and two sine waves as modulation waves. It is the figure represented with the combination of two element numbers to perform, (a) shows a 1st phase period, (b) shows a 2nd phase period. In FIG. 4, the solid line indicates power running operation, the broken line indicates regenerative operation, and the alternate long and short dash line indicates zero output operation. Here, a combination of switching elements that conduct in zero output operation, for example, (a) (S3, S6), the switching elements S3 and S6 that are paired on the positive side and the negative side shown in FIG. The reflux mode is as follows. It can be seen from FIG. 4 that the proposed conventional pulse pattern generation method has 4 commutations in one control cycle (here, corresponding to the number of occurrences of the reflux mode).
[0005]
Even in the conventional method proposed above (hereinafter referred to as “conventional method”), when the conduction ratio is close to 1, narrow pulses are generated. In order to avoid this, a limiter is provided so that narrow pulses of a certain value or less are not generated.
[0006]
[Problems to be solved by the invention]
However, the conventional method has the following problems.
For example, in the case of three phases, as described above, the number of commutations is four in one control cycle, and the number of switching is not minimized. Since the switching loss increases as the number of commutations increases, the switching loss is larger than that when the switching loss is minimized. A higher carrier frequency is preferable for the operating characteristics of the converter circuit, but increases the switching loss, and therefore the carrier frequency cannot be set high.
[0007]
A narrow pulse is likely to be generated when the conduction rate is close to 1 or when the phase period is shifted to the above-described period.
Since a two-phase triangular wave carrier and two sine wave modulation waves are required, the configuration of the analog circuit is not simple.
[0008]
The algorithm is relatively complex and unsuitable for computer control.
In recent years, general-purpose PWM boards that apply PWM pulses to switching elements have become readily available. However, while the conduction period of the switching element occurs twice within one control cycle (for example, (S3, S6) in FIG. 4), the general-purpose PWM board alone does not assume such control. Making a digital circuit does not make it easy to use a general-purpose PWM board.
[0009]
The modulation factor, which is a sine wave modulation wave amplitude ratio to the carrier wave amplitude, takes a value of 1 or less. In order to operate at a flow rate close to 1, the amplitude of the sine wave modulation wave needs to be close to the carrier wave amplitude, that is, the modulation rate needs to be close to 1. When the amplitude of the sine wave modulation wave is close to the carrier wave amplitude, a very narrow pulse is generated. When a pulse having a very narrow width is generated, the generation of such a narrow pulse is limited by the limiter described above. Therefore, there is an upper limit that can increase the modulation wave amplitude, that is, there is an upper limit on the modulation rate, and this upper limit is called the maximum modulation rate. In the conventional method, since the narrow pulse is generated when the amplitude of the sine wave modulation wave is brought close to the carrier wave amplitude, it is difficult to increase the maximum modulation rate.
[0010]
An object of the present invention is to provide a pulse pattern generation method for a three-phase current source converter circuit that overcomes the above-mentioned drawbacks.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is used in a three-phase current source converter circuit having a bridge in which arms having switching elements that are turned on and off by a pulse are connected in pairs in a positive side and a negative side for each phase. The pulse width generation method of the pulse width modulation system has a reflux mode period in which a pair of switching elements included in a pair of arms connected in a pair are simultaneously turned on in a control cycle. The three reflux mode periods are sequentially generated so as to move between adjacent phases, and the switching elements in one positive side arm and one negative side arm having different phases between adjacent reflux mode periods are provided. The pulse pattern is generated so that each switching element is turned on only once during the control period .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.
FIG. 1 shows a three-phase current source converter circuit to which the pulse pattern generation method of the present invention is applied. In FIG. 1, the switching elements S1 to S6 are schematically shown, but are actually any type of switching elements that are turned on and off by pulses such as IGBT and GTO. Although not shown, the switching elements S1 to S6 are usually provided with diodes in order to prevent backflow of current in series. In FIG. 1, 10 indicates a three-phase bridge circuit composed of switching elements S1 to S6, and 12 indicates a three-phase AC power source. Reference numeral 14 denotes a load, which is a superconducting coil in this description. Reference numeral 16 denotes an AC filter for preventing harmonics generated by the switching operation of the three-phase current source converter circuit from flowing out to the three-phase AC power source 12 side.
[0013]
In order to facilitate understanding of the invention, what has been described in the prior art is partially repeated. The PWM pulse pattern is determined by the power supply voltage phase, the three-phase modulation wave conduction ratio command value (in the present invention, the modulation wave is not limited to a sine wave) and the control period. A three-phase alternating current is usually sufficient to express a positive / negative relationship in consideration of a power supply voltage cycle of π / 3. In the following, a phase voltage Va> b phase voltage Vb> 0, c phase voltage Vc = − (Va + Vb) <0/6 period, i.e., first phase period, followed by b-phase voltage Vb> a-phase voltage Va> 0, c-phase voltage Vc =-(Va + Vb) <0, i.e., π / 6 period, i.e., A two-phase period will be described as an example. Here, for simplicity, it is assumed that the conduction ratio command value has the same phase as the power supply voltage so as to realize a power factor of 1, but an arbitrary reactive current can be flowed by controlling the phase of the command value. That is, the information necessary for generating the PWM pulse is only the power supply voltage phase, the three-phase modulation wave conduction ratio command value (Da, Db, Dc), and the control cycle T. A constraint condition for pulse generation in the PWM current source converter is that one of the switching elements on the positive side (S1, S2, S3) and the negative side (S4, S5, S6) must always be on. Under this condition, the pulse generation method of the present invention realizes the order of shifting the on / off mode with the minimum number of switching times in one control cycle in the method in which the narrow pulse is not generated in the region where the output is close to zero. Yes.
[0014]
FIG. 2 is a diagram showing a preferred embodiment of the pulse generation method of the present invention in which the transition sequence of the on / off mode that circulates in the control period in the first and second phase periods is represented by a combination of two element numbers that conduct. Yes, (a) shows the first phase period and (b) shows the second phase period. In FIG. 2, the solid line indicates power running operation, the broken line indicates regenerative operation, and the alternate long and short dash line indicates zero output operation. Here, a combination of switching elements that conducts in zero output operation, for example, (a) (S1, S4), the switching elements S1 and S4 that are paired on the positive side and the negative side shown in FIG. The reflux mode is as follows. In the conventional method, there are two reflux modes between two modes of power running operation (or regenerative operation) within one control cycle (see FIG. 4), whereas in the present invention, there is only one commutation mode. The main feature is that it does not intervene. From FIG. 2, the pulse pattern generation method of the present invention has the minimum number of commutations in one control cycle (here, corresponding to the number of occurrences of the reflux mode) of three times, which is four times that of the conventional method shown in FIG. It can be seen that it is less than once.
[0015]
It should be noted that the on / off mode can be shifted in the direction opposite to the arrow shown in FIG. That is, if the transition direction is selected according to the magnitude relationship between the a and b phases, the control mode may be switched every π / 6 of the power supply phase, and the total number of control modes is 12.
[0016]
Next, the pulse distribution method will be described. The on times Ta and Tb of the modes (S1, S6) and (S2, S6) shown in FIG. 2A are obtained from the a-phase conduction rate command Da and the b-phase conduction rate command Db by the following equations, respectively.
[0017]
[Expression 1]
Ta = T × │Da│
Tb = T × │Db│
The on-time Tc of the c phase automatically becomes a desired value as the sum of the pulse widths Ta and Tb of the a and b phases. That is, the c-phase conduction rate command Dc and the pulse width Tc have the following relationships with the a- and b-phase conduction rate commands Da and Db and the on-time Ta and Tb, respectively.
[0018]
[Expression 2]
│Dc│ = │Da│ + │Db│
Tc = T × | Dc | = T × (| Da | + | Db |)
Therefore, the remaining period Tr of the control cycle T is expressed by the following equation.
[0019]
[Equation 3]
Tr = T− (Ta + Tb) = T−Tc = T × (1− | Dc |)
All the remaining periods Tr are in the reflux mode, but this period is equivalent to the three reflux modes, that is, the modes (S1, S4), (S3, S6) and (S2, S5) shown in FIG. Divide into That is, all the pulse widths Tfw are set to values obtained by the following equation.
[0020]
[Expression 4]
Tfw = Tr / 3 = T × (1- | Dc |) / 3
It will be easily understood that the present invention not only equalizes the periods of the three reflux modes, but can distribute the remaining period Tr in any ratio if desired.
[0021]
The state of this pulse distribution method is shown in FIG. 3 by using a sawtooth wave according to the custom for the first phase period of FIG. 3A shows a PWM switching pattern during power running, FIG. 3B shows a PWM switching pattern during zero output operation, and FIG. 3C shows a PWM switching pattern during regenerative operation. Since Da and Db are negative during the regenerative operation, the mode is changed to (S3, S4), (S3, S5) as shown in FIG. According to the method of the present invention, as can be seen from FIGS. 3A to 3C showing the element ON periods, the ON period exists once in one control period T for all elements. This means that the method of the present invention is suitable for digital control because the subsequent processing can be easily realized by a dedicated PWM board as long as the computer is turned on / off.
[0022]
When the number of commutations within one control cycle of the three-phase current source converter circuit is large, a narrow pulse is likely to be generated in a region where the DC output voltage is low or high. As described above, in the present invention, the number of commutations in one control cycle is as few as three times compared to other conventional methods that do not generate narrow pulses in the region where the output voltage is close to zero. A narrow pulse is not generated in a low region, and it is difficult to generate a narrow pulse even in a high region. Thus, by adopting the on / off mode transition of the switching element that does not generate a narrow pulse even in a region where the DC output voltage is low, the DC output voltage of zero can be realized more easily than the conventional method.
[0023]
Switching loss generally increases as the number of commutations increases. In the present invention, since the number of commutations is the minimum of three, the switching loss due to the number of commutations can be minimized. For example, if the switching loss is proportional to the number of commutations, the present invention does not use a carrier wave as will be described later. However, if expressed using the concept of a carrier wave, the frequency of the carrier wave is 3 minutes compared to the conventional method. Can be set four times higher. Alternatively, in the present invention, the length of one control cycle can be set shorter by three quarters than the conventional method.
[0024]
The present invention determines the conduction rate to determine the length of the total reflux mode period within one control cycle, and then the length of the three reflux mode periods within one control cycle is determined by the total reflux mode period obtained. Can be distributed arbitrarily, and preferably evenly distributed, so that the minimum switching time is not set for each switching element, but the minimum maintenance for the on / off mode of the entire three-phase current source converter is maintained. You can set the time.
[0025]
As described above, the present invention relates to a method for generating a PWM pulse pattern from a conduction rate command value. The method for calculating the duty ratio command value depends on how the converter control system is designed and is not uniquely determined. However, it is obvious to those skilled in the art that once the design policy of the control system is determined, the conduction rate command value corresponding to the desired output current can be obtained using various known methods for calculating the conduction rate command value. I will. The present invention is not limited to the case where the line current flowing into the current source converter (current flowing into the AC filter 16 from the three-phase AC power supply 12 side in FIG. 1) is a sine wave. Similarly to the case of the sine wave, the conduction rate command value obtained in consideration of suppression or the conduction rate command value obtained in consideration of the waveform when the line current is a waveform other than a sine wave. A PWM pulse pattern with features can be generated.
[0026]
Further, in the present invention, when the conduction rate is close to 1, no narrow-width pulse is generated at a value closer to 1 than in the conventional method. Will generate a narrow pulse. In order to avoid this, a limiter can be provided so that the amplitude of the three-phase conduction vector does not exceed the limit value in the higher-order algorithm for determining the conduction ratio command value. In that case, according to the present invention, the limiter can be set so that the limiter operates at a flow rate closer to 1 than in the conventional method.
[0027]
In addition, although the case where the superconducting coil was used was demonstrated about load, this invention may be another type of load and is not limited to the kind of load.
[0028]
【The invention's effect】
Since the present invention is configured as described above, the following effects can be obtained.
In the present invention, the number of commutations is the smallest and the length of each reflux mode period can be arbitrarily distributed including the uniform distribution of all the reflux mode periods. Thus, a wider pulse than the conventional method can be generated, and a narrow pulse is not generated even in a region where the output voltage is low or high compared to the conventional method. Therefore, the present invention does not require the use of a sinusoidal modulated wave or a carrier wave. However, in order to facilitate intuitive understanding, when the modulation rate that is the concept of the conventional method is used, according to the present invention, the three-phase current source converter is It is possible to improve the maximum modulation rate representing the limit operation without generating a narrow pulse in a region where the DC output voltage is high. In other words, the three-phase current source converter of the present invention needs to increase the amplitude of the modulation wave in a region where the DC output voltage is high, but it can be operated without generating a narrow pulse even if this amplitude is increased. be able to.
[0029]
Since the number of commutations is a minimum of three, the switching loss due to the number of commutations can be minimized. Therefore, for example, if the switching loss is proportional to the number of commutations, the present invention does not use a carrier wave, but if expressed using the concept of a conventional carrier wave, the frequency of the carrier wave is 3 minutes compared to the conventional method. Can be set four times higher. In other words, according to the present invention, the length of one control cycle can be set to be 3/4 shorter than that of the conventional method.
[0030]
The algorithm for determining the PWM pulse pattern of the present invention is simpler than the conventional method, is easy to implement with a digital circuit, and is suitable for computer control. Since the digitization is easy and the switching element is turned on only once in one control cycle, the general-purpose PWM board can be easily used.
[0031]
The present invention can also be controlled when the line current flowing into the three-phase current source converter has a waveform other than a sine wave.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a three-phase current source converter circuit.
FIG. 2 shows a transition sequence of an on / off mode that circulates in a control period in the first and second phase periods for one preferred embodiment of the pulse generation method of the present invention applied to the three-phase current source converter circuit shown in FIG. (A) shows a first phase period, and (b) shows a second phase period.
3 is a diagram showing a PWM switching pattern in accordance with the on-off mode transition sequence shown in FIG. 2, wherein (a) is for powering operation, (b) is for zero output operation, and (c) is for regeneration. Each time is shown.
FIG. 4 is a diagram showing a conventional pulse pattern generation method using a two-phase triangular wave in a combination of two element numbers that conducts an on / off mode transition sequence that circulates in a control cycle in the first and second phase periods. (A) shows the first phase period and (b) shows the second phase period.
[Explanation of symbols]
10 Three-phase bridge circuit 12 Three-phase AC power supply 14 Superconducting coil 16 AC filter

Claims (2)

Pulse pattern generation method of pulse width modulation method used in a three-phase current source converter circuit having a bridge in which arms having switching elements that are turned on and off by a pulse are connected in pairs in a positive side and a negative side for each phase In
In the control cycle, there is a reflux mode period in which a pair of switching elements included in the pair of arms connected in pairs are simultaneously turned on, and the three reflux mode periods shift between adjacent phases. And the control circuit has a conduction period for turning on each switching element in one positive arm and one negative arm in different phases between adjacent reflux mode periods, and the control A method of generating a pulse pattern so that each switching element is turned on only once during a period .   The method of claim 1, wherein the three reflux mode periods are substantially equal.

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