CN109756126B - Method for preventing two-level jump of phase voltage of three-level converter - Google Patents
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Abstract
A method for preventing two-level jump of phase voltage of three-level converter. When the three-level converter uses the modulation strategy as carrier reverse phase laminated PWM, two groups of reverse phase laminated triangular carriers with the phase difference of 180 degrees are generated simultaneously; by judging the direction of the three-phase modulation wave, one of the two groups of triangular carriers which can not cause the two-level jump of the phase voltage is selected as an actual comparison triangular carrier when the direction of any phase modulation wave is changed; and comparing the actual comparison triangular carrier wave with the three-phase modulation wave to obtain the switching state of each power device of the three-level converter. The invention can prevent the two-level jump of the phase voltage under the carrier phase inversion lamination PWM of the three-level converter, and improves the safety and the reliability of the three-level converter compared with the traditional situation that the three-level converter uses the carrier phase inversion lamination PWM.
Description
Technical Field
The present invention relates to a PWM control method.
Background
A typical three-level converter, such as a three-level npc (neutral Point clamped) converter, has a topology as shown in fig. 1. The direct-current side voltage of the three-level converter is defined to be 2E, the alternating-current side phase voltage of the three-level converter can respectively output P, O, N three level states by controlling the on-off of each power switch device of a three-phase bridge arm, and the corresponding voltage values are 2E, E and 0 respectively. Compared with a two-level converter, the three-level converter can reduce the voltage stress borne by each power switch device, so that the maximum output voltage can be improved, and the harmonic performance of the three-level converter is more excellent. Based on the advantages, the three-level converter is generally applied to the speed regulation occasion of the medium-high voltage high-power motor at present.
The two-level jump of the phase voltage refers to the direct jump of the alternating-current side phase voltage of the three-level converter from the N level to the P level or from the P level to the N level. For a high-power three-level converter, phase voltage two-level jump is strictly prevented in a steady-state or dynamic operation process, otherwise, the simultaneous action of four series power switching devices of one-phase bridge arm easily causes the damage of unequal dynamic voltage drop of each device, and higher dv/dt is brought. For the pulse width modulation strategy of the high-power three-level converter, the two-level jump of the phase voltage must be prevented.
Carrier-phase-inversion stacked PWM (POD) method, which is one of SPWM methods based on carrier comparison. As shown in fig. 2, the POD method compares two triangular carriers with the same frequency, the same phase, the same amplitude and the like and in opposite directions with a three-phase modulation wave to generate a corresponding PWM control signal, thereby controlling the on/off of each bridge arm power switching device of the three-level converter. The POD method is simple in principle and easy to implement, and is a pulse width modulation strategy which is frequently used.
The POD method generates corresponding PWM control signals by comparing a triangular carrier wave with a three-phase modulation wave, wherein the value of the three-phase modulation wave is a sampling value obtained by regular sampling. In an ideal situation, the sampling zero point of the three-phase modulation wave coincides with the zero point of the triangular carrier, and at this time, no two-level jump of the phase voltage occurs under the POD method, as shown in fig. 3. However, in practical engineering applications, since it is not possible to ensure that the initial sampling value of the three-phase modulation wave is zero, there is a high possibility that the zero point of the modulation wave does not coincide with the zero point of the triangular carrier, and at this time, a two-level jump of the phase voltage may occur under the POD method, as shown in fig. 4.
The three-level converter may have two-level jump of phase voltage under the action of the POD method, so that the safety and the reliability of the three-level converter are reduced, and the POD method is limited to be used in a high-power three-level converter.
Disclosure of Invention
The invention provides a method for preventing two-level jump of phase voltage of a three-level converter, aiming at overcoming the defect that the traditional carrier inversion lamination PWM method is applied to the two-level jump of the phase voltage of the three-level converter. According to the invention, two groups of reverse phase laminated triangular carriers with the phase difference of 180 degrees are generated simultaneously, and one group of the triangular carriers is selected to be compared with the modulation wave when the direction of the modulation wave changes, so that the phase voltage two-level jump is prevented under the action of carrier reverse phase laminated PWM, and the safety and reliability of the three-level converter under the action of carrier reverse phase laminated PWM are improved.
The invention uses an asymmetric regular sampling method for sampling the modulation wave, namely the sampling frequency is twice of the triangular carrier frequency, and the zero point and the peak point of the triangular carrier wave are respectively sampled once.
According to the invention, when the modulation strategy used by the three-level converter is carrier reverse phase laminated PWM, two groups of reverse phase laminated triangular carriers with the phase difference of 180 degrees are simultaneously generated; by judging the direction of the three-phase modulation wave, one of the two groups of triangular carriers which can not cause the two-level jump of the phase voltage is selected as an actual comparison triangular carrier when the direction of any phase modulation wave is changed; the actual comparison of the triangular carrier wave and the three-phase modulation wave is utilized to obtain the switching state of each power device of the three-level converter, and the two-level jump of the phase voltage under the carrier phase inversion lamination PWM of the three-level converter is prevented.
The method comprises the following specific steps:
the three-phase modulated wave of the present invention is defined as follows:
in the formula (1), M is a modulation ratio, ω represents an angular velocity, ω is 2 pi f, and f is a three-phase modulation wave frequency.
The method comprises the following specific steps:
1. simultaneously generate two groups of triangular carriers with phase difference of 180 degrees
Two groups of triangular carriers are simultaneously generated, and each group of triangular carriers is formed by reversely laminating an upper triangular carrier and a lower triangular carrier which have the same amplitude and frequency; the first group of triangular carriers is formed by reversely laminating a triangular carrier Carr1 with an initial value of 0 and an initial direction of a rising direction and a triangular carrier Carr2 with an initial value of 0 and an initial direction of a falling direction, and is defined as follows:
in the definitions of Carr1 and Carr2, fcarrRepresenting the carrier frequency, tcarrCorresponding to a time variable, tcarrThe definition is as follows:
tcarr＝t-S×tcarr(3)
in the formula (3), t corresponds to the actual time, and S is t to tcarrInteger multiples of;
the second group of triangular carriers is formed by reversely laminating a triangular carrier Carr3 with an initial value of 1 and a descending direction as an initial direction and a triangular carrier Carr4 with an initial value of-1 and an ascending direction as an initial direction, and is defined as follows:
2. updating sampling value of three-phase modulation wave when direction of triangular carrier changes
In the invention, the sampling value of the three-phase modulation wave is updated only when the direction of the triangular carrier changes. The method comprises the following steps of defining Carrflagpre as a direction flag bit of a triangular carrier in a last sampling period, and Carrflag as a direction flag bit of the triangular carrier in a current sampling period, wherein the value-taking rule of the Carrflag is as follows:
when Carrflagpre is not equal to Carrflag, the sampling values of the three-phase modulated wave are updated.
3. Determining whether the direction of the three-phase modulated wave is changed
The invention judges whether the direction of the three-phase modulation wave is changed or not according to the change of the sampling value of the three-phase modulation wave. Defining Vapre, Vbpre and Vcpre as A-phase, B-phase and C-phase modulation wave sampling values in the last sampling period respectively, and Va, Vb and Vc as A-phase, B-phase and C-phase modulation wave sampling values in the current sampling period respectively, wherein the specific rule for judging whether the direction of the three-phase modulation wave is changed is as follows:
1) when Vapre <0, Vbpre <0, Vcpre >0 and Va >0 correspond to the A phase direction from negative to positive;
2) when Vapre >0, Vbpre >0, Vcpre <0, Va <0, corresponding to the A phase direction, from positive to negative;
3) when Vapre >0, Vbpre <0, Vcpre <0, Vb >0, the corresponding B phase direction is from negative to positive;
4) when Vapre <0, Vbpre >0, Vcpre >0 and Vb <0 correspond to the direction of the B phase from positive to negative;
5) when Vapre <0, Vbpre >0, Vcpre <0, Vc >0, the corresponding C phase direction is from negative to positive;
6) when Vapre >0, Vbpre <0, Vcpre >0, Vc <0, corresponding to the C phase direction going from positive to negative.
4. When the direction of the three-phase modulation wave is changed, the actual comparison triangular carrier wave is selected from the two groups of triangular carrier waves
The invention selects one of two groups of triangular carriers which can not cause phase voltage two-level jump as an actual comparison triangular carrier when the direction of any phase modulation wave is changed, and the specific selection rule of the actual comparison triangular carrier is as follows:
when the direction of any phase modulation wave is changed, if Carrflag is 0, selecting a first group of triangular carriers Carr1 and Carr2 as actual comparison triangular carriers; if Carrflag is 1, the second set of triangular carriers Carr3 and Carr4 are selected as actual comparison triangular carriers.
5. Actually comparing the sampling values of the triangular carrier wave and the three-phase modulation wave to obtain a PWM signal
The invention obtains the switch state of each power device of the three-level converter by actually comparing the triangular carrier wave with the three-phase modulation wave, and the specific comparison rule is as follows:
1) actually comparing the triangular carriers to a first group of triangular carriers Carr1 and Carr2, and when the sampling value of the three-phase modulation wave is greater than Carr1 and Carr2, outputting a P level corresponding to the phase of the modulation wave; when the sampling value of the three-phase modulation wave is greater than Carr2 and less than Carr1, the corresponding modulation wave phase outputs O level; when the sampling value of the three-phase modulation wave is smaller than Carr1 and Carr2, outputting an N level corresponding to the modulation wave phase;
2) actually comparing the triangular carriers to second group triangular carriers Carr3 and Carr4, and when the sampling value of the three-phase modulation wave is larger than Carr3 and Carr4, outputting P level corresponding to the modulation wave phase; when the sampling value of the three-phase modulation wave is greater than Carr4 and less than Carr3, the corresponding modulation wave phase outputs O level; when the sampling value of the three-phase modulation wave is smaller than Carr3 and Carr4, the corresponding modulation wave outputs N level.
Drawings
FIG. 1 three-level NPC converter topology;
FIG. 2 is a schematic diagram of a carrier-reversed phase stacked PWM method;
FIG. 3 shows that the modulation wave sampling zero point coincides with the triangular carrier zero point under the action of the carrier inversion lamination PWM method, and no phase voltage two-level jumps;
FIG. 4 shows that the modulation wave sampling zero point and the triangular carrier zero point do not coincide under the action of the carrier reverse phase laminated PWM method, and phase voltage two-level jump occurs;
FIG. 5 is a waveform of a three-phase sinusoidal modulation wave in one fundamental period;
fig. 6a and 6b show the zero sampling condition of the 0-degree phase angle modulation wave under the action of the carrier reverse phase stacked PWM method, wherein: FIG. 6a corresponds to a no-phase voltage two-level jump condition, and FIG. 6b corresponds to a phase voltage two-level jump condition;
fig. 7a and 7b show the sampling zero point condition of the 180-degree phase angle modulation wave under the action of the carrier reverse phase stacked PWM method, wherein: FIG. 7a corresponds to a no phase voltage two-level jump condition, and FIG. 7b corresponds to a phase voltage two-level jump condition;
FIG. 8 is a flow chart of an embodiment of the method of the present invention;
fig. 9 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulated wave sample value Ua under the conditions that the modulated wave sample zero coincides with the triangular carrier zero, and the triangular carrier frequency 600Hz in the conventional carrier reverse phase laminated PWM method in the embodiment;
fig. 10 shows that in the conventional carrier-inverted stacked PWM method in the embodiment, the zero point of the modulated wave is not coincident with the zero point of the triangular carrier, the phase voltage Vao is obtained at 600Hz of the triangular carrier frequency, and the sampled values Ua of the triangular carrier Vcarr1, Vcarr2 and the a-phase modulated wave are obtained;
FIG. 11 shows the phase voltage Vao, the triangular carrier Vcarr1, Vcarr2 and the sampling value Ua of the A-phase modulated wave in the embodiment of the method of the present invention when the zero point of the modulated wave is not coincident with the zero point of the triangular carrier wave, and the triangular carrier frequency is 600 Hz;
fig. 12 shows that in the conventional carrier inversion stacked PWM method in the embodiment, the zero point of the modulated wave is not coincident with the zero point of the triangular carrier, the phase voltage Vao is lower than the triangular carrier frequency of 750Hz, and the sampled values Ua of the triangular carrier Vcarr1, Vcarr2 and the modulated wave of a phase are obtained;
fig. 13 shows the phase voltage Vao, the triangular carrier Vcarr1, Vcarr2 and the a-phase modulated wave sample value Ua under the condition that the zero point of the modulated wave sample does not coincide with the zero point of the triangular carrier, and the triangular carrier frequency is 750Hz in the method of the present invention.
Detailed Description
The invention is further described with reference to the following figures and detailed description.
According to the invention, when the modulation strategy used by the three-level converter is carrier reverse phase laminated PWM, two groups of reverse phase laminated triangular carriers with the phase difference of 180 degrees are simultaneously generated; by judging the direction of the three-phase modulation wave, one of the two groups of triangular carriers which can not cause the two-level jump of the phase voltage is selected as an actual comparison triangular carrier when the direction of any phase modulation wave is changed; the actual comparison of the triangular carrier wave and the three-phase modulation wave is utilized to obtain the switching state of each power device of the three-level converter, and the two-level jump of the phase voltage under the carrier phase inversion lamination PWM of the three-level converter is prevented.
The method comprises the following specific steps:
1. simultaneously generate two groups of triangular carriers with phase difference of 180 degrees
The invention relates to a carrier phase inversion lamination PWM (pulse width modulation) device, which only produces one group of triangular carriers, and simultaneously produces two groups of triangular carriers with phase difference of 180 degrees for preventing phase voltage two-level jump.
2. Updating sampling value of three-phase modulation wave when direction of triangular carrier changes
The invention obtains the switch state of each power device by comparing the three-phase modulation wave sampling value with the inverted phase laminated triangular carrier, and the sampling value of the three-phase modulation wave is updated only when the direction of the triangular carrier is changed.
3. Determining whether the direction of the three-phase modulated wave is changed
In order to prevent the phase voltage two-level jump, the invention needs to detect whether the direction of the three-phase modulation wave changes. The three-phase modulated wave changes direction only at zero crossing points of 0 degree and 180 degrees, and the waveform of the three-phase sinusoidal modulated wave in one fundamental wave period is shown in FIG. 5. Defining Vapre, Vbpre and Vcpre as A-phase, B-phase and C-phase modulation wave sampling values in the last sampling period respectively, and defining Va, Vb and Vc as A-phase, B-phase and C-phase modulation wave sampling values in the current sampling period respectively. Observing fig. 5, for phase a at 0 degree phase angle, when Vapre <0, Vbpre <0, Vcpre >0, Va >0, Vb <0, Vc >0, the corresponding phase a modulated wave direction goes from negative to positive; for the phase angle of 180 degrees of A phase, when Vapre >0, Vbpre >0, Vcpre <0, Va <0, Vb >0 and Vc <0, the direction of the corresponding A-phase modulated wave is from positive to negative; for a phase angle of 0 degrees of the B phase, when Vapre >0, Vbpre <0, Vcpre <0, Vb >0, Va >0 and Vc <0, the direction of the corresponding B-phase modulated wave is changed from negative to positive; for a phase angle of 180 degrees of the B phase, when Vapre <0, Vbpre >0, Vcpre >0, Vb <0, Va <0 and Vc >0, the direction of a corresponding B-phase modulated wave is from positive to negative; for a C-phase angle of 0 degrees, when Vapre <0, Vbpre >0, Vcpre <0, Vc >0, Va <0, Vb >0, the direction of the corresponding C-phase modulated wave is from negative to positive; for the phase angle of 180 degrees of C phase, when Vapre >0, Vbpre <0, Vcpre >0, Vc <0, Va >0 and Vb <0, the direction of the corresponding C-phase modulated wave is from positive to negative.
4. When the direction of the three-phase modulation wave is changed, the actual comparison triangular carrier wave is selected from the two groups of triangular carrier waves
The invention selects one of the two groups of triangular carriers which can not cause the jump of two levels of phase voltage as the actual comparison triangular carrier when the direction of any phase modulation wave is changed. In order to correctly select the actual comparison triangular carrier, the condition that phase voltage two-level jump cannot be generated under the action of a carrier phase inversion lamination PWM method is analyzed.
For the carrier reverse phase laminated PWM method, when the zero point of the modulation wave is not coincident with the zero point of the carrier, and the modulation wave corresponds to the falling edge of the upper triangular carrier at the nearest positive sampling value of zero point crossing 0 degree, or corresponds to the rising edge of the lower triangular carrier at the nearest negative sampling value of zero point crossing 180 degrees, phase voltage two-level jump cannot be generated.
The reason why the phase voltage two-level jump cannot be generated on the upper triangular carrier falling edge corresponding to the nearest positive sampling value of 0 degree zero point is analyzed as follows:
as can be seen from fig. 4, when the sampling zero point of the modulation wave does not coincide with the zero point of the triangular carrier, a two-level jump of the phase voltage may occur at the phase angle of 0 degree in the three-level converter under the action of the carrier inversion lamination PWM. All possible modulation wave sampling conditions at a phase angle of 0 degree when the modulation wave sampling zero point and the carrier zero point do not coincide under the action of the carrier reverse phase laminated PWM method are analyzed, and the analysis is summarized in fig. 6a and 6 b. As can be seen from fig. 6a and 6b, when the modulation wave passes through the zero point at 0 degree and the most recently sampled positive value corresponds to the rising edge of the upper triangular carrier, a two-level jump of the phase voltage occurs at 0 degree; when the modulation wave passes through zero point of 0 degree and the latest sampling positive value corresponds to the falling edge of the upper triangular carrier wave, phase voltage two-level jump cannot be generated.
The reason why the rising edge of the lower triangular carrier corresponding to the nearest negative sampling value of 180 degrees passing through the zero point does not generate phase voltage two-level jump is analyzed as follows:
as can be seen from fig. 4, when the sampling zero point of the modulation wave does not coincide with the zero point of the triangular carrier, a phase voltage two-level jump may occur at a phase angle of 180 degrees by the three-level converter under the action of the carrier inversion lamination PWM method. All possible modulation wave sampling conditions at a phase angle of 180 degrees when the modulation wave sampling zero point and the carrier zero point are not coincident under the action of the carrier reverse phase laminated PWM method are analyzed, and the analysis is summarized in fig. 7a and 7 b. As can be seen from fig. 7a and 7b, when the most recently sampled positive value of the modulation wave at 180 degrees crosses zero corresponds to the falling edge of the upper triangular carrier, a two-level jump of the phase voltage occurs at 180 degrees; when the modulation wave is at the nearest sampling negative value of the zero crossing point of 180 degrees and corresponds to the rising edge of the lower triangular carrier, phase voltage two-level jump cannot be generated.
According to the analysis, the three-level converter can not generate phase voltage two-level jump under the action of a carrier inversion lamination PWM method by detecting the nearest positive sampling value at the phase angle of 0 degree and the nearest negative sampling value at the phase angle of 180 degrees of the three-phase modulation wave and then selecting the triangular carrier in the corresponding direction for comparison.
Combining equations (2), (3) and (5), for the first set of triangle carriers Carr1 and Carr2, when Carrflag is 0, Carr1 corresponds to the upper triangle carrier in the falling direction, and Carr2 corresponds to the lower triangle carrier in the rising direction; when Carrflag is 1, Carr1 corresponds to an upper triangular carrier in the rising edge direction, and Carr2 corresponds to a lower triangular carrier in the falling edge direction.
Combining equations (3), (4) and (5), for the second set of triangular carriers Carr3 and Carr4, when Carrflag is 0, Carr3 corresponds to the upper triangular carrier in the rising edge direction, and Carr4 corresponds to the lower triangular carrier in the falling edge direction; when Carrflag is 1, Carr3 corresponds to an upper triangular carrier in the falling direction, and Carr4 corresponds to a lower triangular carrier in the rising direction.
And detecting the nearest positive sampling value at the 0-degree phase angle and the nearest negative sampling value at the 180-degree phase angle of the three-phase modulation wave, namely detecting whether the direction of the three-phase modulation wave changes, wherein the direction corresponds to the 0-degree or 180-degree phase angle. If Carrflag is 0, selecting a first group of triangular carriers Carr1 and Carr2 as actual comparison triangular carriers, Carr1 as an upper triangular carrier in a falling edge direction correspondingly, and Carr2 as a lower triangular carrier in a rising edge direction correspondingly, and comparing the sampled values of Carr1 and Carr2 with the three-phase modulation wave to avoid two-level jump of phase voltage; if Carrflag is 1, the second set of triangular carriers Carr3 and Carr4 are selected as actual comparison triangular carriers, Carr3 corresponds to an upper triangular carrier in the falling direction, and Carr4 corresponds to a lower triangular carrier in the rising direction, and phase voltage two-level jump is not generated by comparing the sampled values of Carr3 and Carr4 with the three-phase modulation wave.
5. Actually comparing the sampling values of the triangular carrier wave and the three-phase modulation wave to obtain a PWM signal
And establishing a comparison rule for actually comparing the triangular carrier wave with the three-phase modulation wave, and comparing the actually compared triangular carrier wave with the three-phase modulation wave according to the comparison rule to obtain the switching state of each power device of the three-level converter, so that the jump of two levels of phase voltage is prevented under the action of a carrier reverse phase laminated PWM method.
The implementation flow of the invention is shown in fig. 8.
According to the invention, two groups of reverse phase laminated triangular carriers with the phase difference of 180 degrees are generated simultaneously, and the group of triangular carriers which can not cause the phase voltage two-level jump is selected to be compared with the modulation wave when the direction of the modulation wave changes, so that the phase voltage two-level jump is prevented under the action of the carrier reverse phase laminated PWM method, the defects of the traditional carrier reverse phase laminated PWM method applied to a three-level converter are overcome, and the safety and reliability of the three-level converter under the action of the carrier reverse phase laminated PWM method are improved.
According to the embodiment of the invention, a three-level NPC inverter model is built by means of PSIM software, the modulation strategy respectively adopts the method for preventing the phase voltage two-level jump and the traditional carrier inversion lamination PWM method, and the effectiveness of the method for preventing the phase voltage two-level jump of the three-level converter provided by the invention is verified by simulation. The simulation conditions are shown in table 1.
Table 1 example simulation experimental conditions
Fig. 9 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulation wave sampling value Ua of the conventional carrier reverse phase laminated PWM method in the embodiment under the condition that the modulation wave sampling zero coincides with the triangular carrier zero and the triangular carrier frequency 600 Hz. Fig. 9 shows that when the sampling zero point of the modulation wave coincides with the zero point of the triangular carrier wave, the phase voltage two-level jump does not occur under the action of the conventional carrier wave inverted phase laminated PWM method.
Fig. 10 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulation wave sampling value Ua of the conventional carrier reverse phase laminated PWM method in the embodiment under the condition that the modulation wave sampling zero point and the triangular carrier zero point are not coincident, and the triangular carrier frequency is 600 Hz. As shown in fig. 10, in the even carrier ratio, when the sampling zero point of the modulation wave does not coincide with the zero point of the triangular carrier, phase voltage two-level jump may occur at the phase angles of 0 degree and 180 degrees under the action of the conventional carrier inversion lamination PWM method, which is not favorable for the safe operation of the three-level converter.
Fig. 11 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulated wave sample value Ua under the condition that the zero point of the modulated wave sample is not coincident with the zero point of the triangular carrier wave and the triangular carrier wave frequency is 600Hz in the embodiment of the invention. Comparing fig. 10 and fig. 11, it can be known that, in the even carrier ratio, when the sampling zero point of the modulation wave and the zero point of the triangular carrier are not coincident, the present invention can effectively prevent the two-level jump of the phase voltage that may occur under the action of the carrier reverse phase laminated PWM method.
Fig. 12 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulation wave sampling value Ua of the conventional carrier reverse phase laminated PWM method in the embodiment under the condition that the modulation wave sampling zero point and the triangular carrier zero point are not coincident, and the triangular carrier frequency is 750 Hz. As shown in fig. 12, in odd carrier ratios, when the sampling zero point of the modulation wave does not coincide with the zero point of the triangular carrier, phase voltage two-level jump may occur at the phase angles of 0 degree and 180 degrees under the action of the conventional carrier inversion lamination PWM method, which is not favorable for the safe operation of the three-level converter.
Fig. 13 shows phase voltage Vao, triangular carrier Vcarr1, Vcarr2 and a-phase modulated wave sample value Ua under the condition that the zero point of the modulated wave sample does not coincide with the zero point of the triangular carrier, and the triangular carrier frequency is 750Hz in the method of the present invention. Comparing fig. 12 and fig. 13, it can be known that, in the odd carrier ratio, when the sampling zero point of the modulation wave and the zero point of the triangular carrier are not coincident, the method of the present invention can effectively prevent the two-level jump of the phase voltage that may occur under the action of the carrier reverse phase laminated PWM method.
As shown in fig. 9 to 13, the results of the embodiment verify the effectiveness of the method for preventing two-level jump of the phase voltage of the three-level converter according to the present invention. When the sampling zero point of the modulation wave is not coincident with the zero point of the triangular carrier wave, under any carrier wave ratio, the method can ensure that the phase voltage two-level jump of the three-level converter does not occur under the action of the carrier wave reverse phase laminated PWM method, thereby improving the safety and the reliability of the three-level converter under the action of the carrier wave reverse phase laminated PWM method.
Claims (3)
1. A method for preventing two-level jump of phase voltage of a three-level converter is characterized in that when the three-level converter uses a modulation strategy as carrier phase inversion lamination PWM, two groups of phase inversion lamination triangular carriers with phase difference of 180 degrees are generated simultaneously; by judging the direction of the three-phase modulation wave, one of the two groups of triangular carriers which can not cause the two-level jump of the phase voltage is selected as an actual comparison triangular carrier when the direction of any phase modulation wave is changed; the actual comparison of the triangular carrier wave and the three-phase modulation wave is utilized to obtain the switching state of each power device of the three-level converter, and the two-level jump of the phase voltage under the carrier phase inversion lamination PWM of the three-level converter is prevented;
defining the direct current side voltage of the three-level converter as 2E, wherein the phase voltage two-level jump means that the phase voltage level is directly changed from 2E to 0 or from 0 to 2E;
the two groups of triangular carriers with the phase difference of 180 degrees are respectively formed by reversely laminating an upper triangular carrier and a lower triangular carrier which have the same amplitude and frequency; the first group of triangular carriers is formed by reversely laminating a triangular carrier Carr1 with an initial value of 0 and an initial direction of a rising direction and a triangular carrier Carr2 with an initial value of 0 and an initial direction of a falling direction, and is defined as follows:
in the definitions of Carr1 and Carr2, fcarrRepresenting the carrier frequency, tcarrCorresponding to a time variable, tcarrThe definition is as follows:
tcarr＝t-S×tcarr
for tcarrIn the definition, t corresponds to the actual time, and S is t to tcarrInteger multiples of;
the second group of triangular carriers is formed by reversely laminating a triangular carrier Carr3 with an initial value of 1 and a descending direction as an initial direction and a triangular carrier Carr4 with an initial value of-1 and an ascending direction as an initial direction, and is defined as follows:
the three-phase modulated wave is defined as follows:
in the above formula, M is a modulation ratio, ω represents an angular velocity, ω is 2 pi f, f is a three-phase modulation wave frequency, and U isa、Ub、UcIs a three-phase modulated wave, in which UaFor A-phase modulated waves, UbModulating waves, U, for phase BcIs a C-phase modulation wave;
judging whether the direction of the three-phase modulation wave changes or not according to the change of the sampling value of the three-phase modulation wave; defining Vapre, Vbpre and Vcpre as A-phase, B-phase and C-phase modulation wave sampling values in the last sampling period respectively, and Va, Vb and Vc as A-phase, B-phase and C-phase modulation wave sampling values in the current sampling period respectively, wherein the specific rule for judging whether the direction of the three-phase modulation wave is changed is as follows:
1) when Vapre <0, Vbpre <0, Vcpre >0 and Va >0 correspond to the A phase direction from negative to positive;
2) when Vapre >0, Vbpre >0, Vcpre <0, Va <0, corresponding to the A phase direction, from positive to negative;
3) when Vapre >0, Vbpre <0, Vcpre <0, Vb >0, the corresponding B phase direction is from negative to positive;
4) when Vapre <0, Vbpre >0, Vcpre >0 and Vb <0 correspond to the direction of the B phase from positive to negative;
5) when Vapre <0, Vbpre >0, Vcpre <0, Vc >0, the corresponding C phase direction is from negative to positive;
6) when Vapre >0, Vbpre <0, Vcpre >0, Vc <0, corresponding to the C phase direction going from positive to negative.
2. The method for preventing two-level jump of phase voltage of three-level converter according to claim 1, wherein the sampling value of said three-phase modulation wave is updated only when the direction of the triangular carrier changes; the method defines Carrflagpre as the direction flag bit of the triangular carrier wave in the last sampling period, Carrflag as the direction flag bit of the triangular carrier wave in the current sampling period, and comprises the following steps:
when Carrflagpre is not equal to Carrflag, the sampling values of the three-phase modulated wave are updated.
3. The method for preventing two-level jump of phase voltage of three-level converter according to claim 1 or 2, wherein the group of two groups of triangular carriers which does not cause two-level jump of phase voltage is selected as the actual comparison triangular carrier when the direction of any phase modulation wave changes, and the specific selection rule of the actual comparison triangular carrier is as follows:
when the direction of any phase modulation wave is changed, if Carrflag is 0, selecting a first group of triangular carriers Carr1 and Carr2 as actual comparison triangular carriers; if Carrflag is 1, selecting a second group of triangular carriers Carr3 and Carr4 as actual comparison triangular carriers;
in the above selection rule, Carrflag is a direction flag bit of the triangular carrier in the current sampling period.
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