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Emc test and EMI suppression of frequency converter


  1 Introduction

  With the miniaturization, multi-function and high performance of the frequency converter, especially the full digitalization of the control means, the flexibility and adaptability of the frequency converter are constantly increasing. Therefore, in modern industry, frequency converters are used more and more widely. At present, almost all inverters use pwm (pulse width modulation, pwm pulse width modulation technology) control technology.

  The motor system driven by pwm frequency conversion converts and controls the electric energy through the power converter, and the performance index of the motor system is greatly improved. However, since the power switching device in the inverter works in the switching state, the high-speed switching action of the device makes the voltage and current jump in a short time, which makes the voltage and current contain rich high-order harmonics. The electromagnetic of these harmonics Noise energy will form electromagnetic interference (EMI) through circuit connection or electromagnetic wave space coupling, which will have a great impact on the motor system itself and the surrounding environment [1-4]. In the generated conducted interference, the noise signal frequency ranges from a few khz to tens of mhz, and the interference intensity may far exceed the limit value stipulated by the electromagnetic compatibility standard.

  Therefore, designers of modern electronic and electrical systems such as frequency converters are faced with a new challenge of "how to ensure that the electronic and electrical systems can achieve the design purpose in the electromagnetic environment where they are located, and at the same time do not interfere with the normal operation of other electrical systems around them". The problem is the electromagnetic compatibility (electromagnetic compatibility, emc) problem of electronic and electrical systems.

  At present, the international community attaches great importance to electromagnetic compatibility issues, and several organizations have been established to formulate electromagnetic compatibility standards, such as the cispr standard of the International Radio Special Committee, the iec series standards, and the en series standards of the European Union. In China, many organizations such as the National Radio Interference Committee, the Electromagnetic Compatibility Committee of the China Power Supply Society, and the Electromagnetic Compatibility Branch of the IEEE Beijing Branch are also engaged in work related to emc. my country's national emc standards have also been formulated and implemented, such as the national standard (gb) and the military standard (gjb), all of which have promoted the research and development of electromagnetic compatibility in our country. The electromagnetic interference produced by the pwm variable frequency motor drive system has also been paid more and more attention by people. In order to meet the requirements of electromagnetic compatibility standards, correct design and reasonable use of suppression means can reduce the emission intensity of the system's emi to below the limit of the emc standard, so that electrical equipment and systems can achieve electromagnetic compatibility.

  2 Conducted interference mechanism of pwm inverter

  The so-called conductive coupling refers to the energy of electromagnetic noise in the form of voltage or current in the circuit, through metal wires or other components (such as capacitors, inductors and transformers, etc.) coupled to the disturbed equipment (circuit). Conductive coupling can be divided into direct conductive coupling and common impedance conductive coupling. Direct conductive coupling means that the noise is directly coupled to the harassed equipment (circuit) through actual components such as wires, metal bodies, resistors, capacitors, inductors, and transformers. Common impedance conductive coupling refers to the common ground impedance coupling generated by the noise through the printed circuit board circuit and the chassis ground wire, the public safety ground wire of the equipment, and the common ground impedance in the ground network; the noise passes through the common ground impedance of the AC power supply and the DC power supply. When the source impedance is high, common source impedance coupling occurs.

  The switching operation state of the power switching device causes complex mutual coupling among the components in the system, which will form conduction interference. The highest frequency considered for conducted interference is 30mhz, and the corresponding electromagnetic wave wavelength λ in vacuum is 10m. Therefore, for power electronic devices with a size smaller than λ/2π, they belong to the near-field range, and lumped parameter circuits can be used for electromagnetic interference analysis. For the convenience of analysis, the disturbance on the input/output wires of the system can be divided into two parts: common-mode interference and differential-mode interference according to the different transmission and coupling channels of the conducted interference. It is generally believed that the common-mode interference is mainly due to the power semiconductor in the system converter. The dv/dt caused by the switching action of the switching device propagates through the stray capacitive coupling of the system to the ground, and the voltage change of one pole will be capacitively coupled to the other pole to generate displacement current. The current generated by the parasitic capacitance does not require a direct electrical connection, or even ground [5]. Its size can be expressed as:

  i=cdu/dt

  In the formula, c is the equivalent coupling capacitance between the battery interference source and the sensitive equipment.

  Differential mode interference is mainly due to the di/dt caused by the switching of power semiconductor switching devices through the conductor between the input and output lines. Of course, these are only the most essential causes of conducted interference, and different motor systems have different specific causes of conducted interference. In addition, common-mode interference and differential interference can be transformed into each other, and they are not absolutely separated. For example, Figure 1 shows the circuit diagram of the non-essential differential-mode noise caused by the unbalance of the common-mode current transmission channel [6].

  Figure 1 The mechanism of non-essential differential mode noise

  Figure 2 is a diagram of the electromagnetic interference current flow path of the pwm variable frequency drive motor system, including common-mode interference and differential-mode interference. In the pwm inverter, in order to ensure that the switch tube will not fail due to overheating when it is working, a radiator must be installed on it, and in order to prevent short circuit, the metal casing of the switch tube and the radiator are isolated by a heat-conducting insulating medium , and the radiator is grounded through the chassis, so a large parasitic capacitance is formed between the inverter and the radiator [7,8]. When the inverter is working normally, as the upper and lower switching tubes of each phase bridge arm are turned on in turn, the potential of the midpoint of the bridge arm will undergo a quasi-step change. If you look at this phenomenon from the perspective of emi, then the voltage output by the three bridge arms is the three emi interference sources, and each switching action will charge and discharge the parasitic capacitance between the power switching device and the heat sink, forming a common mode emi current.

  Fig. 2 Electromagnetic interference current flow path diagram of pwm variable frequency drive motor system

  3 Restraining Measures of Conducted Disturbance of pwm Inverter

  Since electromagnetic interference must have three elements: electromagnetic interference source, electromagnetic interference propagation path and sensitive equipment, so to suppress the conduction interference of pwm variable frequency drive motor system must also start with three elements, that is, reduce the intensity of interference source, cut off the propagation path and Improve noise immunity of sensitive equipment.

  3.1 Emi suppression technology based on reducing the emission intensity of interference sources

  From the perspective of reducing the intensity of interference sources, there are three representative methods: changing the circuit topology, improving the control strategy and optimizing the drive circuit.

  (1) Change the circuit topology

  The idea of ​​improving the circuit topology is mainly to eliminate the common-mode voltage of the converter output through a symmetrical structure, and reduce the level of conducted interference emission on the input side of the device because the voltage change rate on the switching device is halved. Scholars headed by aljulian proposed a three-phase four-leg scheme [9-11] for eliminating the output common-mode voltage of three-phase power converters based on the principle of "circuit balance". The experimental circuit is shown in Figure 3. The basic idea of ​​this method is to use an additional "auxiliary phase" to make the ground potential of the three-phase system circuit symmetrical, and by adjusting the switching sequence, the sum of the output phase voltages of the four bridge arms is as zero as possible, and the common-mode voltage is completely zero. . Compared with traditional three-leg power converters, its common-mode emi can be reduced by about 50%.

  Figure 3 Three-phase four-leg power converter with second-order filter

  m.d.manjrekar和a.rao等学者提出了一种通过添加辅助零状态开关,以消除因零开关状态而产生共模电压的方案[12,13],电路结构见图4所示。这种辅助零状态合成器方法在经济方面很有吸引力,并且还可以使消除感应电机侧共模电压。

  图4 辅助零状态合成器结构图

  与传统的功率变换相比,尽管三相四桥臂和辅助零状态合成器这两种方法都能够消除或降低系统的共模电压,但它们所采用的调制策略都会使系统电压利用率下降。为此,haoran zhang等学者提出了一种用于消除电机共模电压和轴电流的双桥功率变换器[14-16],拓扑结构见图5所示。它是通过控制双桥功率变换器产生标准的三相双绕组感应电动机平衡激励,并通过平衡激励(磁系统)实现抵消共模电压,达到消除轴电压、轴电流及充分减小漏电流、emi发射强度的目的。

  图5 双功率变换器驱动电路

  为了消除pwm电机驱动系统共模电流,a.consoli等学者基于共模电压补偿技术,提出了一种应用于由两个或多个功率变换器组成的多驱动系统公共直流母线共模电流消除技术[17],拓扑结构见图6所示。该方法是在两个功率变换器做适当连接的基础上,通过控制两个变换器状态序列而使共模电压同步变化的新pwm调制策略。

  图6 公共直流母线多电动机驱动共模电压抑制系统

  (2)改进控制策略

  由于两电平pwm调制策略将不可避免的使功率变换器输出含有共模电压,为此一些学者基于改进逆变器控制方式或策略,提出了一些可以消除或减小共模电压的新调制策略。如台北学者yen-shi lai所提出的空间矢量调制技术(space-vector pwm,svpwm),该方法是利用矢量状态的不同组合会对功率变换器输出共模电压产生影响的特点,采用两个相反方向矢量“回扫”的方法取代了零矢量的作用,以降低系统共模电压,实现抑制传导emi的目的[18,19]。而a.m.de broe等学者提出了整流侧与逆变侧开关同步变化的空间矢量调制方法[20],它能够避免产生与直流母线电压大小相同的共模电压脉冲;韩国学者hyeoun-dong lee对全控型三相整流/逆变器的空间矢量调制方式进行了改动[21],它是依据非零矢量位置的移动会减小系统输出共模电压脉冲数量和作用时间这一原理,实现共模电压的减小。另外该学者还提出了通过检测整流器滤波电容钳位中点电位的过零点极性,并选用两个不同零矢量的方法。该方法可以将功率变换器输出的共模电压降低到传统svpwm方式的三分之二[22];再有m.zigliotto等学者提出了以随机开关频率调制(random pulse width modulation,rpwm)方式实现电磁干扰能量在频域范围内分布平均化的抑制技术[23]。

  (3)优化驱动电路

  The main reason for the conduction emi generated by the pwm motor drive system is that the dv/dt and di/dt caused by the high-frequency switching action of the power semiconductor device are too large, and their size also directly affects the emission intensity of the system emi, and for commonly used For switching devices, the magnitude of dv/dt and di/dt at the moment of switching is affected by the gate drive pulse waveform and gate stray capacitance [24]. Appropriate circuit topology and control strategy can reduce dv/dt and di/dt, and realize the reduction of system emi emission intensity. Based on this point of view, Japanese scholar s.takizawa and Italian scholar a.consoli realized the controllability of the gate drive current waveform by adding a drive current source, and achieved the purpose of optimizing emc [25,26]. Scholars such as vinod john proposed a three-level driving idea according to the structural characteristics, switching characteristics and the Maitreya effect of the igbt [27], and designed the corresponding circuit. It can be applied to both discrete devices and igbt modules, and it is also suitable for soft switching and hard switching technologies; another way to reduce dv/dt and di/dt is to increase the buffer absorption circuit. This method reduces the dv/dt and di/dt to a certain extent, and has an effect on improving the system emi, but in fact it only eliminates the oscillation phenomenon (burr phenomenon) when the device is switched, and the effect is not very obvious.

  Scholars such as p.caldeira proposed that the soft switching converter using zero voltage transition (zvt) should be more efficient than the hard switching converter emi Good performance speculates [28].

  3.2 The emi suppression method based on cutting off the conduction transmission path

  Although purely from the perspective of EMC, reducing the intensity of electromagnetic interference emitted by interference sources can reduce the system emi, but it will be limited by unfavorable factors such as increased switching loss, limited suppression range, complicated control strategies, and reduced voltage utilization. For this reason, scholars from various countries have successively proposed some emi filter structures for blocking emi propagation paths, and experiments have shown that properly designed filters can reduce the emi emission intensity of the system to below the emc standard limit, which is an important factor for electrical equipment It is an important means to achieve electromagnetic compatibility with the system. Like harmonic filters, emi filters can also be divided into passive emi filters and active emi filters.

  (1) Active emi filter

  Active filters are used to eliminate emi noise energy through active circuits. The specific working principle of the active filter is to detect the emi current or voltage through the detection link, and then feed it back to the system, so as to offset the emi current or voltage generated by the system and achieve the purpose of eliminating emi.

  Active filters currently used in PWM motor drive systems are mainly used to eliminate common-mode components in conducted emi [29,30]. A typical active filter used to eliminate common mode current is shown in Figure 7. It consists of a small common-mode current transformer and a pair of complementary high-frequency transistors. When the inverter switches, the high-frequency leakage current returns to the power supply side through the parasitic capacitance between the motor winding and the frame, and the common-mode current transformer The common-mode current isl is detected and amplified by the complementary transistor to generate a compensation current il′. If the product of the transformer ratio and the transistor amplification factor is large enough, the leakage current il can be eliminated, and the common-mode current isl flowing into the power supply side is completely suppressed.

  Figure 7 Active filter for eliminating common mode current

  The traditional active filter used to eliminate common-mode voltage is shown in Figure 8, which is called active common-mode noise canceller (ACC) in [30], ACC is connected to the output terminal of the inverter and three cables Among them, it is composed of a common-mode voltage sensor, a compensation circuit and a common-mode transformer. ACC superimposes a compensation voltage at the output of the inverter. The compensation voltage is opposite in polarity and equal in amplitude to the common-mode voltage generated by the pwm inverter. Thus, the common-mode voltage applied to the load is completely eliminated, which reduces the common-mode current and conducted emi.

  Figure 8 Active filter used to remove common-mode voltage

  (2) Passive emi filter

  Passive emi filters are usually composed of components such as resistors, inductors, and capacitors. At present, the most common one is the power emi filter, and its structure is shown in Figure 9. Because it can only suppress emi noise, but has no effect on other negative effects of the pwm motor drive system, scholars from various countries have successively proposed some passive emi filters that take into account other functions. For example, the common-mode transformer scheme proposed by scholars such as avjouanne [31], the structure is shown in Figure 10. This scheme is designed from the perspective of eliminating the common mode emi current on the motor side. It is based on the common mode choke coil, and then winds a fourth winding terminal connected to the damping resistor on the same core to suppress the common mode emi current. The oscillation of the mode emi current can eliminate other negative effects caused by the common mode voltage at the motor end.

  Figure 9 Typical three-phase emi power filter

  Figure 10 Common mode transformer scheme

  Scholars such as darendusara proposed an improved second-order rlc low-pass power converter output filter [32,33], the structure shown in Figure 11. Compared with the prototype filter, its important difference is that the neutral point "n`" of the resistance-capacitance circuit connected in star form is connected with the neutral point "m" of the DC bus clamp of the converter through wires. The advantage of this filter is that it can reduce the conduction differential mode emi current and conduction common mode emi current on the motor side at the same time, and if the parameters are designed properly, the values ​​of rf, lf and cf can also be made small, and it is installed in the power inside the converter housing. It can significantly reduce the overvoltage at the motor terminal, common mode emi current to ground and shaft voltage, and the size, loss and cost of the filter are all low.

  Figure 11 Improved second-order passive low-pass filter

  4 Conclusion

  The electromagnetic compatibility of frequency converter is an increasingly serious problem with the rapid development of power electronics technology and electronic integration technology and the continuous increase of system capacity in recent decades. With the enforcement of international standards and the continuous emergence of new technologies in the process of scientific research The electromagnetic interference problem of the frequency converter makes the electromagnetic compatibility problem of the frequency converter an urgent problem to be solved. This article starts with the analysis of the conduction interference mechanism of the pwm frequency converter, and summarizes the current suppression measures for conduction interference, which is of reference significance. Generally speaking, the electromagnetic compatibility design of the frequency converter is still in the initial stage, and we need to pay long-term and unremitting efforts.

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