CN109600067B - Uniformly-distributed PWM method and system suitable for three-phase power electronic converter - Google Patents
Uniformly-distributed PWM method and system suitable for three-phase power electronic converter Download PDFInfo
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- CN109600067B CN109600067B CN201811391814.7A CN201811391814A CN109600067B CN 109600067 B CN109600067 B CN 109600067B CN 201811391814 A CN201811391814 A CN 201811391814A CN 109600067 B CN109600067 B CN 109600067B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/44—Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
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- Electromagnetism (AREA)
- Inverter Devices (AREA)
- Dc-Dc Converters (AREA)
Abstract
The invention discloses a uniformly distributed PWM method and a uniformly distributed PWM system suitable for a three-phase power electronic converter, and belongs to the technical field of power electronics. Compared with the conventional modulation method of fixed switching frequency, the invention updates the change of the switching frequency according to the change of the electrical angle of the three-phase system, and the statistical characteristic of the designed time domain waveform of the switching frequency meets the rule of uniform distribution and also considers the principle of reducing the switching loss. In addition, the best raw switching frequency suitable for optimizing converter output performance using the present invention is also indicated. Compared with the conventional modulation method of fixed switching frequency, the invention can reduce conducted EMI to the maximum extent in the frequency band of 150kHz-5MHz and reduce the volume of an EMI filter to improve the power density through the change of the switching frequency; meanwhile, the switching loss generated by the converter can be reduced, and the system efficiency is improved.
Description
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a uniformly distributed PWM (pulse width modulation) method suitable for a three-phase power electronic converter.
Background
The PWM type power electronic converter is widely used in many fields of home, business and industry, such as solar photovoltaic power generation, offshore wind power generation, high-speed rail traction, uninterruptible power supply, flexible dc power transmission, etc. However, the application of the pulse width modulation technique brings about large electrical stress (dv/dt and di/dt), which may generate conducted electromagnetic interference to affect the normal operation of the equipment or even reduce the reliability of the system. At present, power electronic equipment is continuously developing towards high voltage, large capacity, high switching frequency and high power density, and especially, the application of high switching frequency broadband semiconductor devices in the power electronic field further deteriorates conducted EMI. Therefore, corresponding measures must be taken to limit the emission level of conducted EMI within the range specified by the standard.
The main methods for suppressing the electromagnetic interference generated by the power electronic converter are to block or bypass the propagation path of the electromagnetic interference and to reduce the noise source. An EMI (Electromagnetic Interference) filter is an effective method for suppressing conducted EMI on a propagation path, and a good EMI filter design can make the conducted EMI meet the standard requirement. However, the introduction of passive components greatly increases the weight and volume of the system.
Therefore, reducing the source of EMI noise can significantly reduce the conducted EMI emission level of the equipment without adding additional hardware. For conventional fixed switching frequency PWM modulation strategies, the harmonic energy is mainly concentrated near the carrier harmonics. Mapping onto the spectrum of conducted EMI has many spikes near the carrier frequency that severely degrade the electromagnetic interference. Based on this, many scholars develop PWM strategies that vary the switching frequency. Most representative are Random PWM (RPWM) and model-based predictive Variable Switching Frequency PWM (VSFPWM). RPWM randomly varies the switching frequency over a range without taking into account switching losses and current ripple. VSFPWM varies the switching frequency by controlling ripple based on real-time prediction of ripple, and has been applied to applications such as capacitor ripple control of rectifiers, torque ripple control of motor systems, and the like. However, this modulation method is dependent on the operating conditions of the converter, and in many operating conditions, the suppression of conducted EMI is not good.
Disclosure of Invention
In view of the above drawbacks and needs of the prior art, the present invention provides a uniformly distributed PWM method for a three-phase power electronic converter, which aims to reduce conducted electromagnetic interference and switching loss, thereby solving the technical problem of degrading conducted electromagnetic interference by applying a high switching frequency silicon carbide converter.
To achieve the above object, according to one aspect of the present invention, there is provided a uniformly distributed PWM method for a three-phase power electronic converter, comprising the steps of:
(1) extracting the electrical angle ω t of the three-phase system, and carrying out the following transformation according to the magnitude of the electrical angle ω t:
(2) substituting the transformed ω T into a variation expression T (ω T) of the switching period to obtain the switching period of the next carrier period, thereby controlling the real-time variation of the switching frequency;
wherein the coefficients
k is a switching frequency variation coefficient;
(3) and the real-time change of the switching frequency is realized by controlling the size of the switching period in the next carrier period.
According to another aspect of the present invention there is provided a uniformly distributed PWM system for a three-phase power electronic converter, comprising the following modules:
the phase-locked loop and conversion module is used for extracting the electrical angle omega t of the three-phase system and carrying out the following conversion according to the size of the electrical angle omega t:
the switching period calculation module is used for substituting the transformed ω T into a variation expression T (ω T) of the switching period to obtain the switching period of the next carrier period;
wherein the coefficients
k is a switching frequency variation coefficient;
and the control module is used for controlling the real-time change of the switching frequency through the switching period.
Further, the switching frequency variation coefficient
fswFor the original fixed switching frequency, f, of a three-phase power electronic convertersw_lowThe switching frequency is changed to be the lower limit of the switching frequency.
Further, if the switching frequency variation coefficient k is greater than 0.6, k is made 0.6.
In general, the above technical solutions contemplated by the present invention are compared with the prior art:
compared with the conventional modulation method of fixed switching frequency, the invention can reduce conducted EMI to the maximum extent in the frequency band of 150kHz-5MHz and reduce the volume of an EMI filter to improve the power density through the change of the switching frequency; meanwhile, the switching loss generated by the converter can be reduced, and the system efficiency is improved.
Furthermore, the invention also indicates the optimal original fixed switching frequency suitable for optimizing the output performance of the converter by adopting the invention, and because the frequency spectrum range of the conducted EMI and the switching speed and the loss of power electronic devices in practical application are considered, when the original switching frequency is about 50kHz, the proposed uniformly distributed PWM algorithm is adopted, the effect of optimally reducing the conducted EMI is achieved, and the invention has good technical effects of reducing the external conducted interference and the volume of an EMI filter and the like.
Drawings
FIG. 1 is a three-phase two-level converter in general;
FIG. 2 is a graph of the effect of a fixed switching frequency on reducing conducted EMI for a variable switching frequency, where FIG. 2a is a graph of the conducted EMI for a modulation strategy with a fixed switching frequency of 10kHz versus the corresponding VSFPWM, and FIG. 2b is a graph of the conducted EMI for a modulation strategy with a fixed switching frequency of 50kHz versus the corresponding VSFPWM;
FIG. 3 is a schematic diagram of a linear variation of the switching frequency;
FIG. 4 is a reference sinusoidal modulation wave of a three-phase converter;
fig. 5 is the switching frequency waveform (calculated result) of the proposed uniform PWM;
fig. 6 is a statistical distribution of switching frequencies for the proposed uniformly distributed PWM (experimental results);
FIG. 7 is a comparison of conducted EMI for conventional modulation methods and proposed uniformly distributed PWM (experimental results);
FIG. 8 is a comparison of switching losses (calculated) for the conventional modulation method and the proposed uniformly distributed PWM;
fig. 9 is a block diagram of an implementation of the proposed uniform distribution PWM.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The present invention is a modulation method based on switching frequency uniform distribution to reduce conducted EMI and switching losses developed based on a general three-phase two-level converter as shown in fig. 1. The converter can be operated in rectifier mode. It is also possible to operate in inverter mode. The three-phase current-intersecting side can be connected with a power grid or provided with three-phase loads such as a motor and the like.
Fig. 2 is a graph illustrating the effect of switching frequency on reducing conducted EMI using a variable switching frequency approach. For fixed switching frequency modulation strategies, double fourier analysis shows that the harmonic energy is mainly concentrated near the carrier harmonics, especially the first few carrier harmonics. Fig. 2a is a conducted EMI spectrum at different switching frequencies. It can be seen that for a switching frequency of 50kHz, there are many sharp peaks in the spectrum. This means that the energy distribution between the carrier harmonics and their side-band harmonics is extremely uneven, and the gap is much smaller in the case of a switching frequency of 10 kHz. Because the starting point of the spectrum 150kHz is the 3 rd carrier harmonic for 50kHz and already the 15 th carrier harmonic for 10kHz, the carrier harmonic at this point has decayed rapidly to almost no longer be clearly identified from its carrier cluster than the first few carrier harmonics. For example, for 50kHz and 10kHz, the difference between the maximum and minimum values of electromagnetic interference around 150kHz of the spectrum is 70dB and 35dB, respectively. In other words, the more non-uniform the distribution of harmonic energy across the spectrum, the greater the ability to vary the switching frequency to reduce EMI. The conducted EMI after the switching rate is changed is also shown in FIG. 2, and it is obvious that the conducted EMI reducing effect is better in the case of 50 kHz. Therefore, to achieve a better conducted EMI reduction, the fixed switching frequency should be chosen such that the dominant carrier harmonics are concentrated at a switching frequency around 150kHz, such as around 50 kHz.
The derivation of the analytic expression of the switching frequency of the uniformly distributed PWM is analyzed below. It is assumed that the switching frequency varies linearly (either linearly increasing or linearly decreasing) within a given variation range. Fig. 3 is a diagram illustrating a linear variation of the switching frequency. The sampling points at two moments are respectively at A1And B1A is1With lower switching frequency, i.e. with longer switching period, and B1At higher switching frequencies, i.e. shorter switching periods, i.e. atA>ΔtBTo obtain the next sampling point A respectively2And B2. Is obviously at A1The number of sampling points near the frequency is less than that at B1The number of samples near the switching frequency, resulting in the switching frequency still being concentrated at high switching frequencies, the spike in fig. 2 still occurs. In order to distribute the switching frequency uniformly, the waveform of the switching frequency should satisfy the following law: the rate of change is smaller at low switching frequencies and larger at high switching frequencies. The following equation can satisfy this rule:
where ω represents the fundamental frequency (rad/s), f is the time domain expression of the switching frequency, and C is a constant.
The switching frequency can be uniformly distributed by satisfying the above formula, so that the conducted EMI can obtain more attenuation.
It is another object of the invention to reduce switching losses. Fig. 4 shows the three-phase reference modulated wave and its maximum absolute value (i.e., the envelope of 6 pulses as shown). For most converters operating at unity power factor, the current and voltage are in phase, and the three phase currents are substantially similar to the voltages of fig. 4 except for magnitude. For the switching loss, the larger the absolute value of the current, the larger the switching loss. Therefore, to reduce switching losses, the higher the current, the lower the switching frequency should be. For a three-phase system, the switching frequency should be inversely related to the maximum of the absolute values of the three-phase currents. I.e. the same period as the six-pulse envelope in fig. 4, anti-monotonicity. The periodicity and symmetry are summarized as follows, and in one fundamental period (i.e. ω t is in the range of [0,2 π ]), the following characteristics are satisfied
(1) The periodicity is as follows: the repetition period of the switching frequency is one sixth of the fundamental period, i.e. pi/3 rad
(2) Symmetry: ω t is in the range of [0, π/3], and the waveform of the switching frequency is symmetrical about t π/6
Due to the periodicity and symmetry of the switching frequency, the analytical formula is only required to be solved within [0, pi/6 ]. The derivative is carried out on the ω t at two sides of the switching frequency expression given in the foregoing to obtain
f”(ωt)f(ωt)-(f'(θ)ωt)2＝0
The above equation is a second order differential equation, and two boundary conditions are needed to solve. The upper and lower limits of the switching frequency variation are then given as two boundary conditions:
f(0)＝fsw_upper＝fsw
thus, an expression for the switching frequency is found:
in the expression, the solving of the index takes a large amount of operation time of the controller; in addition, the switching frequency of the DSP is controlled by the switching period, and the division operation also increases the burden. Therefore, after taking the reciprocal of the above formula, the first 5 terms are taken after the above formula is expanded by the maculing formula, and the following can be obtained:
wherein the coefficient a is
Fig. 5 shows an accurate graph of the switching frequency variation and a fitted curve thereof, and it is clear that the two almost completely coincide without any difference, demonstrating that the approximate fit is extremely accurate.
Fig. 6 shows a statistical distribution of the switching frequency obtained in the experiment, which is substantially uniform, and in view of this, this method is named uniform distribution PWM.
Fig. 7 is a graph comparing conducted EMI of the proposed uniformly distributed PWM and conventional PWM, and clearly shows better attenuation of EMI at low frequency band with few sharp peaks. Particularly at 150kHz, where EMI is worst, the attenuation of EMI can reach nearly 20 dB.
Fig. 8 is a comparison of switching losses, and it is evident that the proposed uniformly distributed PWM also greatly reduces switching losses and improves system efficiency.
Combining the above analysis and description, fig. 9 shows a block diagram of an implementation of the proposed uniform distribution PWM.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (4)
1. A uniformly distributed PWM method suitable for a three-phase power electronic converter is characterized by comprising the following steps:
(1) extracting the electrical angle ω t of the three-phase system, and carrying out the following transformation according to the magnitude of the electrical angle ω t:
(2) substituting the transformed ω T into a variation expression T (ω T) of the switching period to obtain the switching period of the next carrier period:
wherein the coefficients
k is a switching frequency variation coefficient;
the switching frequency variation coefficient
fswFor the original fixed switching frequency, f, of a three-phase power electronic convertersw_lowThe switching frequency is the lower limit of the switching frequency after the switching frequency is changed;
(3) and the real-time change of the switching frequency is realized by controlling the size of the switching period in the next carrier period.
2. The uniformly distributed PWM method according to claim 1, wherein if the switching frequency variation coefficient k is greater than 0.6, then k is 0.6.
3. A uniformly distributed PWM system suitable for a three-phase power electronic converter, comprising the following modules:
the phase-locked loop and conversion module is used for extracting the electrical angle omega t of the three-phase system and carrying out the following conversion according to the size of the electrical angle omega t:
the switching period calculation module is used for substituting the transformed ω T into a variation expression T (ω T) of the switching period to obtain the switching period of the next carrier period;
wherein the coefficients
k is a switching frequency variation coefficient;
the switching frequency variation coefficient
fswFor the original fixed switching frequency, f, of a three-phase power electronic convertersw_lowThe switching frequency is the lower limit of the switching frequency after the switching frequency is changed;
and the control module is used for controlling the real-time change of the switching frequency through the switching period of the next carrier wave period.
4. The uniformly distributed PWM system according to claim 3, wherein if the coefficient of variation of the switching frequency, k, is greater than 0.6, then k is 0.6.
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| US5790390A (en) * | 1994-08-05 | 1998-08-04 | Kayser Ventures, Ltd. | Power supply with reduced EMI |
| JP2004159418A (en) * | 2002-11-06 | 2004-06-03 | Canon Inc | Switching power supply |
| US7177166B1 (en) * | 2005-08-30 | 2007-02-13 | Microchip Technology Incorporated | Pulse width modulation frequency dithering in a switch mode power supply |
| WO2011016199A1 (en) * | 2009-08-05 | 2011-02-10 | 三菱電機株式会社 | Dc/dc power converter |
| CN106208729B (en) * | 2015-05-25 | 2020-01-21 | 松下知识产权经营株式会社 | Power conversion device |
| CN106385196A (en) * | 2016-09-27 | 2017-02-08 | 华中科技大学 | Current ripple real-time prediction model-based three-level voltage source variable switching frequency control method |
| CN106787918B (en) * | 2017-01-16 | 2019-05-14 | 南京航空航天大学 | A kind of random SVPWM method of Five-phase inverter |
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