Common-mode EMI noise suppression method combining passive filter and modulation strategy

Abstract

The invention discloses a common-mode EMI noise suppression method combining a passive filter and a modulation strategy, and belongs to the technical field of power electronics and electricians. According to the method, an improved random switching frequency SVPWM (Space Vector Pulse Width Modulation) method is used, a common-mode voltage frequency spectrum without an EMI (Electro-Magnetic Interference) filter is obtained through measurement, a common-mode voltage amplitude is reduced from the source, the reduction of a common-mode electromagnetic interference frequency spectrum is further realized, required insertion loss is determined according to an EMC (Electro Magnetic Compatibility) standard, and an intersection point of the insertion loss and the minimum frequency is reached. And taking the frequency corresponding to the point as the cut-off frequency of the EMI filter, determining the element parameters of the filter according to the cut-off frequency, and verifying the element parameters. By using a common-mode interference comprehensive suppression means combining the filter and the random PWM strategy, the design margin of the filter can be reduced, the cut-off frequency of the EMI filter can be increased, the weight and the size of the inductor can be reduced, the light weight and the miniaturization of the filter can be realized, and the cost can be reduced. And meanwhile, the improved random switching frequency SVPWM is adopted, so that the filtering effect of the EMI filter is guaranteed.

Description

Technical Field

[0001] The present invention relates to the fields of power electronics technology and electrical engineering technology, and in particular to a common-mode EMI noise suppression method combining a passive filter and a modulation strategy. Background Art

[0002] The rapid action of the switching devices of the power converter in the motor control system can cause electromagnetic interference problems, which can be divided into differential mode electromagnetic interference and common mode electromagnetic interference. Common mode electromagnetic interference is caused by common mode current. Common mode current will generate common mode voltage and form a loop through the phase line and the ground. It will flow through the bearing through the coupling capacitor path inside the motor, induce a high shaft voltage on the motor shaft, and form a bearing current. The bearing current will discharge through the oil film, continuously damage the bearing surface, and easily cause aging and damage to the motor.

[0003] At present, the electromagnetic interference suppression methods of motor control systems can be mainly divided into two categories. One is to reduce the emission intensity of EMI interference sources; the other is to cut off the EMI conduction path. Among them, the EMI suppression technologies that have received more attention mainly include: EMI filter technology, switching waveform optimization technology and improved modulation strategy.

[0004] EMI filter technology can be divided into three categories: active, passive and hybrid filters, all of which suppress electromagnetic interference current from the conduction path. Passive EMI filters block EMI conduction paths through passive components such as inductors and capacitors, but have poor low-frequency performance and require the addition of inductors and capacitors that are larger in size and weight. Active EMI filters detect EMI interference signals through active devices such as operational amplifiers and power amplifiers, and inject opposite compensation signals, but are constrained by high-frequency parasitic parameters, compensation response speed and system stability requirements. Hybrid EMI filters combine the advantages of both but are complex in design. Switching waveform optimization technology suppresses EMI by controlling the voltage and current change rates of switching devices, but may affect the switching loss and efficiency of the system, including soft switching technology and active gate drive technology. The improved modulation strategy starts with pulse width modulation, with the goal of reducing the amplitude of the noise source, and conducts a series of improvements and innovations in the modulation method. The related methods include random pulse width modulation (RPWM) and variable switching frequency pulse width modulation (VSFPWM). Reducing the common mode voltage amplitude from the source is conducive to improving the electromagnetic interference situation, but it cannot guarantee that the EMI current of the entire system will be reduced. At the frequency point with worse resonance, the impedance value is very small, and the EMI current does not decrease but the amplitude may increase.

[0005] Therefore, in order to better suppress common-mode electromagnetic interference at high frequencies, it is necessary to further study a common-mode EMI noise suppression method that can reduce the filter size without affecting the suppression effect.

[0006] This patent proposes a common-mode EMI noise suppression method that combines a passive filter and a modulation strategy to address the problem that traditional passive filter methods lead to increased core size and filter weight and volume. Summary of the invention

[0007] In view of the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a common-mode EMI noise suppression method combining a passive filter and a modulation strategy, by adopting a common-mode interference comprehensive suppression method combining a filter and a random PWM strategy, and improving the design method of a common-mode EMI filter on the basis of adopting a random PWM strategy. After adopting the improved random switching frequency SVPWM, the design margin of the filter can be reduced and the cutoff frequency of the EMI filter can be increased, which is conducive to reducing the weight and volume of the inductor, realizing the lightweight and miniaturization of the filter, and at the same time realizing the electromagnetic interference suppression of the servo drive system at a relatively low cost. This method effectively solves the problems of excessive core size, weight and volume of the filter brought by the traditional passive EMI filter, and ensures the suppression effect of common-mode electromagnetic interference.

[0008] The present invention adopts the following technical solutions to achieve the above-mentioned purpose:

[0009] A common-mode EMI noise suppression method combining passive filters and modulation strategies. The common-mode EMI noise suppression design process adopted is as follows:

[0010] Step 1.1, using the improved random switching frequency SVPWM instead of the traditional random switching strategy SVPWM, and measuring the common mode voltage spectrum without adding the EMI filter, reducing the common mode voltage amplitude from the source, and realizing the reduction of the common mode electromagnetic interference spectrum;

[0011] Step 1.2: Determine the required insertion loss according to the EMC standard, and obtain the intersection point where the two frequencies are the smallest. The frequency corresponding to this point is used as the corner frequency value required for the filter element parameter design.

[0012] Step 1.3: Design an LCL type EMI filter and obtain the key component parameters: common mode capacitor C Y and common mode inductor L CM The parameter values ​​are obtained and verified. If they meet the standards, the overall design of the filter is completed.

[0013] Step 1.4: If the standard is not met, the cutoff frequency is changed from nf s Reduced to (n-1)f s , repeat the above design steps and get the cutoff frequency (n-1)f s The corresponding LCL type EMI filter component parameters are checked again and the adjustment is repeated until the final EMC standard is met.

[0014] Furthermore, the random PWM strategy adopted by the method is an improved random switching frequency SVPWM. Under the condition that the spread spectrum range is 5% to 10% of the switching frequency, the method can further disperse the high-frequency harmonics and reduce their peak values, thereby achieving the purpose of suppressing electromagnetic interference; the improved random switching frequency SVPWM strategy divides the original spread spectrum range into n frequency bands, each frequency band corresponds to a new average switching frequency, and sets the switching frequency switching period T b , every time T b The switching frequency change band is switched once every / n time, thereby reducing the distribution range of the switching frequency value differences between adjacent switching cycles while keeping the overall spread spectrum range and the average switching frequency unchanged.

[0015] Furthermore, the original noise of the converter is measured using the LISN circuit. The noise attenuation required by the system is obtained by measuring the noise before the EMI filter is added. The common-mode and differential-mode interferences calculated by the measurement are compared with the limit values ​​required by the electromagnetic compatibility standard to obtain the insertion loss IL required by the designed common-mode EMI filter. REQ The formula is as follows:

[0016] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV)

[0017] Where V measure That is, the common-mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin The safety margin is appropriately increased according to the actual situation. According to GJB 151B-2013 Requirements and Measurements of Electromagnetic Emission and Sensitivity in Military Equipment and Subsystems CE102 Standard, A Limit It is a limit value in dBμV (decibel microvolt) that is used to specify the maximum allowable conducted emission level of a device within a specific frequency range to ensure that the device does not generate excessive electromagnetic interference during operation and affect the normal operation of other devices.

[0018] Furthermore, the calculated insertion loss can be used to obtain the minimum frequency point where the system common mode noise intersects the peak standard line given by the EMC standard. The nearest switching frequency multiple k is assigned to n, and f req_min With nf s In comparison, the definition of the turning frequency f c =nf s -Δf / 4, thus obtaining the switching frequency multiple k and the turning frequency f c The relationship between the magnitude Δf of the RSF-SVPWM spread spectrum is used to determine the component parameters of the common-mode EMI filter.

[0019] Furthermore, when the transition frequency is constant, according to the capacitance requirements under the power level specified in "GJB151B-2013", and taking into account the reduction of the volume of the entire EMI filter, a Y capacitor close to 0.075μF / kW is selected, and the typical value of the Y capacitor is 330nF.

[0020] Furthermore, the common mode inductor L CM The value of the common mode capacitor C Y With the turning frequency f c The values ​​of are related to the common mode inductance L CM The value of can be given by the following formula:

[0021]

[0022] Compared with the prior art, the technical solution adopted by the present invention has the following beneficial effects:

[0023] (1) By using the improved random switching frequency SVPWM strategy modulation, this method can further disperse high-frequency harmonics and reduce their peak values ​​under the condition of a small switching frequency variation range. Compared with the random PWM strategy, it solves the problem that it is difficult to balance the high-order harmonic dispersion effect and system control performance;

[0024] (2) In order to meet the EMI standard, the traditional common-mode EMI filter increases the number of inductor turns, which leads to an increase in the size of the magnetic core and the weight and volume of the filter. By improving the design method of the common-mode EMI filter based on the use of a random PWM strategy, it is beneficial to reduce the weight and volume of the inductor and achieve lightweight and miniaturization of the filter.

[0025] (3) In order to solve the problem that the traditional modulation method cannot guarantee the system reliability and the filter core size is too large, the modulation method and the EMI filter method are combined by using a comprehensive common-mode interference suppression method that combines the filter and the random PWM strategy. The additional attenuation provided by the modulation method can avoid the increase in the weight and volume of the filter inductor to a certain extent, and can also provide an additional guarantee for the EMI filter to achieve the filtering effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of a common-mode EMI noise suppression method combining a passive filter and a modulation strategy in the present invention.

[0027] Figure 2 It is a schematic diagram of a circuit for measuring interference voltage on LISN in the present invention.

[0028] Figure 3It is a circuit diagram of an LCL type EMI filter used for suppressing common mode EMI noise in the present invention.

[0029] Figure 4 It is a schematic diagram of system EMI common mode noise when no EMI filter is used for suppression in the present invention.

[0030] Figure 5 It is a schematic diagram of the system EMI common mode noise after being suppressed by the EMI filter in the present invention. DETAILED DESCRIPTION

[0031] The technical scheme of the present invention is further described below based on the embodiment, but the present invention is not limited to the specific embodiment. In the detailed description of the present invention below, some specific details are described in detail. For those skilled in the art, the present invention can be fully understood without the description of these details.

[0032] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings:

[0033] A common-mode EMI noise suppression method combining a passive filter and a modulation strategy, the method uses an improved random switching frequency SVPWM modulation method and measures the common-mode voltage spectrum without adding an EMI filter, reduces the common-mode voltage amplitude from the source, and then reduces the common-mode electromagnetic interference spectrum, and determines the required insertion loss according to the EMC standard, to the intersection point where the two frequencies are the smallest, and uses the frequency corresponding to this point as the cutoff frequency of the EMI filter, and then determines the filter element parameters according to the cutoff frequency. By using a common-mode interference comprehensive suppression method combining a filter and a random PWM strategy, the design margin of the filter can be reduced and the cutoff frequency of the EMI filter can be increased, which is conducive to reducing the weight and volume of the inductor, and realizing the lightweight and miniaturization of the filter. At the same time, the use of the improved random switching frequency SVPWM also provides an additional guarantee for the EMI filter to achieve the filtering effect.

[0034] The common mode EMI noise suppression design process used in the method is as follows Figure 1 As shown, it includes using the improved random switching frequency SVPWM to replace the traditional random switching strategy SVPWM to obtain a certain amount of common mode EMI spectrum reduction instead of the original unsuppressed common mode noise; determining the required insertion loss according to the EMC standard, and obtaining the minimum intersection of the two frequencies, and obtaining the corner frequency value required for the next step of filter component parameter design; then designing the LCL type EMI filter, and obtaining the key component parameter C Y With L CM The parameter values ​​are determined and verified to ensure the rationality and reliability of the filter design.

[0035] In order to obtain a reduced common-mode noise spectrum, the random PWM strategy adopted by the method is an improved random switching frequency SVPWM. Under the condition that the switching frequency variation range is small, this method can further disperse high-frequency harmonics and reduce their peak values, thereby achieving the purpose of suppressing electromagnetic interference. The change amount of the random numbers taken from adjacent switching cycles is used to reflect the change level of the random sequence, and the random number R of the kth switching cycle value is determined. k for:

[0036]

[0037] Furthermore, the high-order harmonic amplitude and the random number R used to randomly modulate the switching frequency are used. k By analyzing the influence of random number sequence characteristics on the amplitude of high-order harmonics, it can be found that the random number difference sequence R taken from adjacent switching cycles i+1 -R i The smaller the distribution range of , the smaller the amplitude of the high-order harmonics. Therefore, the improvement of the traditional RSF-SVPWM strategy can be achieved by reducing the distribution range of the random number difference sequence taken from adjacent switching cycles while keeping the spread spectrum range the same, dividing the original spread spectrum range into n frequency bands, each frequency band corresponds to the new average switching frequency, and setting the switching frequency switching period T b , every time T b The switching frequency change band is switched once every / n time, thereby reducing the distribution range of the switching frequency value difference between adjacent switching cycles while keeping the overall spread spectrum range and average switching frequency unchanged, further optimizing the high-order harmonic dispersion effect. Figure 4 and Figure 5 It can be seen from the system EMI common mode noise that, compared with the traditional SVPWM strategy and the improved random switching frequency SVPWM strategy, the system common mode noise peak under the improved random switching frequency SVPWM strategy is significantly reduced. That is, based on this, the problem that the traditional RSF-SVPWM is difficult to balance the high-order harmonic dispersion effect and system control performance is improved. Under the same spread spectrum range, better harmonic dispersion effect is obtained, which can improve the system EMC performance without sacrificing the comprehensive control performance of the PMSM servo system.

[0038] Furthermore, before selecting the EMI filter topology, it is necessary to first measure the interference noise when no EMI filter is added to the inverter. Figure 2As shown in the figure, the original noise of the converter is measured using LISN, that is, the noise attenuation required by the system is obtained by measuring the noise before the EMI filter is added, and the common mode interference calculated by the measurement is compared with the limit value required by the electromagnetic compatibility standard, and a 6dB margin is reserved, which is the insertion loss required for the designed common mode EMI filter. Before the EMI filter is added, the interference voltage V at the two test ports is measured at the same time. 1 and V 2 , and then separate the time domain signal measured on LISN into differential mode and common mode, and the common mode voltage V flowing through LISN CM It can be calculated by the following formula:

[0039]

[0040] According to GJB151B-2013 electromagnetic emission and sensitivity requirements in military equipment and subsystems and the EMC peak standard lines given by CE101 and CE102, the application object of CE101 standard is the common mode current from 25Hz to 10kHz, and the application object of CE102 standard is the common mode voltage from 10kHz to 30MHz. The required insertion loss IL can be calculated by subtracting the specified limit from the common mode noise. REQ , where V measure That is, the common-mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin The safety margin value is appropriately increased according to the actual situation. Margin Taking 6dB, the insertion loss in the present invention is calculated to be 50dB.

[0041] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV)

[0042] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV) = 44dB + 6dB = 50dB

[0043] Further, such as Figure 4The system EMI common-mode noise shown in the figure shows that compared with the traditional RSF-SVPWM strategy, the system common-mode EMI noise peak obtained by the improved random switching frequency SVPWM strategy has been significantly reduced. Under the same spread spectrum range, better harmonic dispersion effect is obtained. The insertion loss can be used to obtain the minimum frequency point where the system common-mode noise intersects with the peak standard line given by the EMC standard. The nearest switching frequency multiple k is assigned to n, and f req_min With nf s In comparison, the definition of the turning frequency f c =nf s -Δf / 4, where Δf represents the frequency spread range of RSF-SVPWM, and the value of Δf can be half of the switching frequency multiple k, which is used to determine the filter component parameters. The filter selects LCL type filter. When designing EMI filter, it is usually designed for the common mode and differential mode components in the interference signal respectively. Therefore, the parameter value C of the differential mode filter is X The value of 2μF is selected and will not be elaborated here. The following only designs the component parameter value C of the common mode EMI filter. Y With L CM .

[0044] Furthermore, the volume of the EMI filter mainly depends on the size of the common-mode inductor. Therefore, when the transition frequency is constant, a larger Y capacitor should be selected as much as possible to obtain a smaller common-mode inductor, thereby reducing the volume of the entire EMI filter. The size of the Y capacitor is restricted by the ground leakage current. The size of the leakage current is closely related to human safety. The leakage current of all power electronic devices has certain limitations. According to the requirements for safety capacitors in "GJB151B-2013": for 50Hz AC power equipment, it should be less than 0.1μF; for 400Hz AC power equipment, it should be less than 0.02μF; for DC power supply systems, the capacitance of each polarity power line to the ground at the interface should not exceed 0.075μF / kW of the connected load. For DC loads less than 0.5kW, the filter capacitance should not exceed 0.03μF. The rated power of the servo drive system is 5kW, so C Y Should not be greater than 375nF. Therefore, select C Y is 330nF.

[0045] Furthermore, the two coils of the common-mode inductor are wound in the same direction on the magnetic ring, and the magnetic field generated by the common-mode current in the two coils is in the same direction, which has a strong inhibitory effect on the common-mode current. The value of the common-mode inductor is related to the value of the common-mode capacitor. The common-mode inductor L CM The value of can be given by the following formula.

[0046]

[0047] Theoretically, the larger the common-mode inductor, the better. However, when designing an EMI filter, the filter volume must be considered, so the common-mode inductor cannot be too large. At the same time, considering that the high-frequency characteristics of the common-mode inductor may affect the performance of the EMI filter, combined with the common-mode capacitor C given above, Y and the turning frequency f c The value of L is taken because the corner frequency of the LC common mode filter should not be higher than 13.9kHz. CM =220μH, at this time the LC common mode filter corner frequency is 13.21kHz.

[0048] Further, such as Figure 5 The figure shows the result of common-mode noise suppression after adding the common-mode EMI filter. The common-mode noise of the system after adding the filter is measured again and compared with the peak standard line given by the EMC standard to determine whether it passes the EMC standard. If it meets the standard, the overall design of the filter is completed; if it still does not meet the standard, the cutoff frequency is increased from nf s Reduced to (n-1)f s , repeat the above design steps and get the cutoff frequency (n-1)f s The corresponding LCL type EMI filter component parameters are verified again and the adjustment is repeated until the final EMC standard is met to ensure the rationality and reliability of the filter design. Figure 5 It can be seen that the common-mode noise after using the improved random switching frequency SVPWM strategy is suppressed to below the EMC standard value, while the traditional RSF-SVPWM strategy has an unsatisfactory suppression effect in the low frequency band, exceeding the standard value by up to about 16dB. After adopting the improved random switching frequency SVPWM, the design margin of the filter can be reduced, and its cut-off frequency value can be increased, which is conducive to reducing the weight and volume of the inductor, realizing the lightweight and miniaturization of the filter, and realizing the electromagnetic interference suppression of the servo drive system at a lower cost. Combining the active suppression method with the passive suppression method, the additional attenuation provided by the active suppression method can avoid the increase in the weight and volume of the filter inductor to a certain extent, and can also provide an additional guarantee for the EMI filter to achieve the filtering effect.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A common mode EMI noise suppression method combining passive filter and modulation strategy, characterized in that: The common-mode EMI noise suppression design process adopted by the method is as follows: Step 1.1, using the improved random switching frequency SVPWM instead of the traditional random switching strategy SVPWM, and measuring the common mode voltage spectrum without adding the EMI filter, reducing the common mode voltage amplitude from the source, and realizing the reduction of the common mode electromagnetic interference spectrum; Step 1.2: Determine the required insertion loss according to the EMC standard, and obtain the intersection point where the two frequencies are the smallest. The frequency corresponding to this point is used as the corner frequency value required for the filter element parameter design. Step 1.3: Design an LCL type EMI filter and obtain the key component parameters: common mode capacitor C Y and common mode inductor L CM The parameter values ​​are obtained and verified. If they meet the standards, the overall design of the filter is completed. Step 1.4: If the standard is not met, the cutoff frequency is changed from nf s Reduced to (n-1)f s , repeat the above design steps and get the cutoff frequency (n-1)f s The corresponding LCL type EMI filter component parameters are checked again and the adjustment is repeated until the EMC standards are finally met.

2. A common mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: The random PWM strategy adopted by the method is an improved random switching frequency SVPWM. Under the condition that the spread spectrum range is 5% to 10% of the switching frequency, the method can further disperse the high-frequency harmonics and reduce their peak values, thereby achieving the purpose of suppressing electromagnetic interference. The improved random switching frequency SVPWM strategy divides the original spread spectrum range into n frequency bands, each frequency band corresponds to a new average switching frequency, and sets the switching frequency switching period T b , every time T b The switching frequency change band is switched once every / n time, thereby reducing the distribution range of the switching frequency value differences between adjacent switching cycles while keeping the overall spread spectrum range and the average switching frequency unchanged.

3. The common mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: The original noise of the converter is measured using the LISN circuit. The noise attenuation required by the system is obtained by measuring the noise before the EMI filter is added. The common-mode and differential-mode interference calculated by the measurement are compared with the limit values ​​required by the electromagnetic compatibility standard to obtain the insertion loss IL required by the designed common-mode EMI filter. REQ The formula is as follows: THE REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV) Among them, V measure That is, the common-mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin A is the safety margin value appropriately increased according to actual conditions; Limit It is a limit in dBμV (decibel microvolt) that specifies the maximum permissible conducted emission level of a device within a specific frequency range.

4. A common mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 3, characterized in that: The insertion loss can be used to obtain the minimum frequency point where the system common mode noise intersects with the peak standard line given by the EMC standard. The nearest switching frequency multiple k is assigned to n, and f req_min With nf s In comparison, the definition of the turning frequency f c =nf s -Δf / 4, thus obtaining the switching frequency multiple k and the turning frequency f c The relationship between the magnitude Δf of the RSF-SVPWM spread spectrum is used to determine the component parameters of the common-mode EMI filter.

5. The common mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: For the selection of safety capacitors, under the condition of a certain transition frequency, according to the capacitance requirements under the power level specified in "GJB151B-2013", and taking into account the reduction of the volume of the entire EMI filter, a Y capacitor close to 0.075μF / kW is selected, and the typical value of the Y capacitor is 330nF.

6. A common mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: The common mode inductor L CM The value of the common mode capacitor C Y With the turning frequency f c The values ​​of are related to the common mode inductance L CM The value of can be given by the following formula:

Images