Variable-frequency drive (VFD) is a power control device that controls the AC motor by changing the working frequency of the motor by applying variable frequency technology and microelectronic technology. In the industrial field, the interference problem of the inverter appears more and more serious, and even the control system cannot be put into normal use. For example, the PLC communication control becomes unstable, for example, the indicator light of the field control cabinet is always bright, which is misleading. Users are very distressed because of the troubles caused by inverter interference. However, the operating principle of the frequency converter is destined to generate strong electromagnetic interference.
In various industrial control systems, with the widespread use of power electronic devices such as inverters, the electromagnetic interference (EMI) of the system is becoming more and more serious, and the corresponding anti-interference design technology (ie, electromagnetic compatibility EMC) has become more and more important. The interference of the inverter system can sometimes directly cause damage to the hardware of the system. Sometimes, although the hardware of the system cannot be damaged, the system program of the microprocessor is often out of control, resulting in control failure, resulting in equipment and production accidents. Therefore, how to improve the anti-interference ability and reliability of the system is an important part of the development and application of automation devices, and it is also one of the keys to the application and promotion of computer control technology. When talking about the anti-interference problem of the inverter, we must first understand the source of the interference and the mode of propagation, and then take different measures for these interferences.
The inverter includes a rectifier circuit and an inverter circuit. The input AC power is converted into a DC voltage through a rectifier circuit and a smooth wave circuit, and then the DC voltage is converted into a pulse voltage of a different width by the inverter (referred to as a pulse width modulation voltage, PWM). . By using this PWM voltage to drive the motor, it is possible to adjust the torque and speed of the motor. This principle of operation leads to the following three types of electromagnetic interference:
(1) Radio frequency interference: RF radiation interferes with the input and output cables from the frequency converter. In the case of the above-mentioned radio frequency conducted emission interference, when there is radio frequency interference current on the input and output cables of the frequency converter, since the cable is equivalent to the antenna, electromagnetic wave radiation is inevitably generated, and radiation interference is generated. The PWM voltage transmitted on the output cable of the inverter also contains abundant high-frequency components, which will generate electromagnetic wave radiation and form radiation interference. The characteristic of radiated interference is that the interference phenomenon becomes serious when other electronic devices are close to the inverter.
(2) Harmonic interference: The rectifier circuit generates harmonic current, which generates a voltage drop in the impedance of the power supply system, causing distortion of the voltage waveform. This distortion voltage interferes with many electronic devices (because Most electronic devices can only operate under sinusoidal voltage conditions. A common voltage distortion is the flattening of the top of a sine wave. When the harmonic current is constant, the voltage distortion is more serious in the case of a weak power supply, which is characterized by interference with equipment using the same power grid, regardless of the distance between the equipment and the frequency converter;
(3) RF conducted emission interference: Since the load voltage is pulsed, the inverter draws current from the grid as a pulse. This pulse current contains a large amount of high-frequency components to form radio frequency interference. Interference with equipment using the same grid, regardless of the distance between the equipment and the frequency converter.
1. Source of inverter interferenceThe first is interference from the external grid.
Harmonic interference in the power grid mainly interferes with the frequency converter through the power supply of the frequency converter. There are a large number of harmonic sources in the power grid, such as various rectification equipment, AC and DC interchange equipment, electronic voltage adjustment equipment, non-linear loads and lighting equipment. These loads cause waveforms and voltages in the power grid to be distorted, which can cause harmful interference to other equipment in the power grid. If the power supply of the inverter is disturbed by the contaminated AC grid, the grid noise will interfere with the inverter through the grid power supply circuit. The interference of the power supply has a main over-voltage, under-voltage, instantaneous power-down b surge, drop c-peak voltage pulse d radio frequency interference.
(1) Interference of thyristor converter equipment to the inverter
When there is a large capacity thyristor commutation device in the power supply network, since the thyristor is always turned on for a part of the time in each phase of the phase, it is easy to cause a notch in the network voltage, and the waveform is seriously distorted. It makes the rectifier circuit on the input side of the inverter likely to be damaged by the large reverse recovery voltage, which causes the input circuit to break down and burn.
(2) Power compensation capacitor interference to the inverter
The power sector has certain requirements on the power factor of the power unit. For this reason, many users use the centralized capacitor compensation method to improve the power factor in the substation. During the transient process of the compensation capacitor input or cut-out, the network voltage may have a very high peak value, and as a result, the rectifier diode of the inverter may be broken by being subjected to excessive reverse voltage.
Second is the interference of the inverter itself to the outside.
The rectifier bridge of the frequency converter is a non-linear load to the power grid, and the harmonics generated by it generate harmonic interference to other electronic and electrical equipment of the same power grid. In addition, most of the inverters of the inverter use PWM technology. When operating in the switching mode and switching at high speed, a large amount of coupling noise is generated. Therefore, the frequency converter is an electromagnetic interference source to other electronic and electrical equipment in the system.
The input and output currents of the inverter contain many high-order harmonic components. In addition to the lower harmonics that make up the power supply's reactive losses, there are many high-frequency harmonic components. They will spread their energy in various ways to form an interference signal to the frequency converter itself and other equipment.
(1) Input current waveform The input side of the inverter is a diode rectification and capacitor filter circuit. Obviously, only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor, there is a charging current in the rectifier bridge. Therefore, the charging current always appears near the amplitude value of the power supply voltage, in the form of a discontinuous shock wave. It has a strong high harmonic component. The relevant data shows that the harmonic components of the 5th harmonic and 7th harmonic in the input current are the largest, which are 80% and 70% of the 50HZ fundamental wave, respectively.
(2) Output voltage and current waveforms Most inverters of inverters adopt SPWM modulation mode, and their output voltages are series of rectangular waveforms whose duty cycle is distributed according to sinusoidal law; due to the inductive nature of motor stator windings, The current of the stator is very close to a sine wave. However, the harmonic component equal to the carrier frequency is still large.
The frequency converter can generate harmonics with large power. Because of the large power, it has strong interference to other equipments of the system. The interference path is consistent with the general electromagnetic interference approach. The main conduction is (ie, circuit coupling), electromagnetic radiation, and induction. coupling. Specifically: firstly, electromagnetic radiation is generated to surrounding electronic and electrical equipment; secondly, electromagnetic noise is generated to the directly driven motor, so that the iron loss and copper consumption of the motor are increased; and the interference is transmitted to the power source, and is transmitted to other equipment of the system through the power distribution network; Finally, the inverter inductively couples adjacent lines to induce interference voltage or current. Similarly, the interfering signals in the system interfere with the normal operation of the inverter through the same route.
(1) The circuit coupling method is transmitted through the power network. Since the input current is non-sinusoidal, when the capacity of the inverter is large, the network voltage will be distorted, affecting the work of other equipment, and the conducted interference generated at the output will greatly increase the copper loss and iron loss of the directly driven motor. Affects the operating characteristics of the motor. Obviously, this is the main mode of propagation of the variable frequency input current interference signal.
(2) Inductive coupling mode When the input circuit or output circuit of the inverter is close to the circuit of other equipment, the higher harmonic signal of the inverter will be coupled to other equipment by induction. There are two ways to sense:
a electromagnetic induction mode, which is the main way of current interference signal;
b Electrostatic induction, which is the main way of voltage interference signals.
(3) The airborne radiation is radiated into the air by electromagnetic waves, which is the main mode of propagation of harmonic components with high frequency.
3. Anti-interference measures of frequency conversion speed control systemAccording to the basic principle of electromagnetic, electromagnetic interference (EMI) must have three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-interference is the most basic and most important anti-interference measure of the application of the system. Generally, the anti-jamming and anti-interference measures are used to suppress interference. The general principle is to suppress and eliminate the interference source, cut off the interference channel to the system, Reduce the sensitivity of system interference signals. Specific measures can be isolated, filtered, shielded, grounded, etc. in the project.
(1) The so-called interference isolation means that the interference source and the susceptible part are isolated from the circuit so that they do not have an electrical connection. In the variable frequency speed control transmission system, an isolation transformer is usually used on the power line between the power supply and the amplifier circuit to avoid conducted interference, and the power isolation transformer can be applied with a noise isolation transformer.
(2) The function of setting the filter in the system line is to suppress the interference signal from the inverter through the power line conduction interference to the power supply from the motor. In order to reduce electromagnetic noise and loss, an output filter can be set on the output side of the inverter; to reduce interference to the power supply, an input filter can be set on the input side of the inverter. If there are sensitive electronic devices in the line, a power supply noise filter can be placed on the power line to avoid conducted interference. In the input and output circuits of the inverter, in addition to the above lower harmonic components, there are many high-frequency harmonic currents, which will propagate their energy in various ways to form interference to other devices. signal. Filters are the primary means of attenuating higher frequency harmonic components. According to the location of use, it can be divided into:
There are usually two types of input filters:
a Line filter: mainly composed of inductor line diagram. It attenuates higher frequency harmonic currents by increasing the impedance of the line at high frequencies.
b Radiation filter: mainly composed of high frequency capacitors. It will absorb harmonic components with high frequency and radiant energy.
The output filter is also composed of an inductor coil. It can effectively attenuate higher harmonic components in the output current. It not only acts as an anti-interference, but also weakens the additional torque caused by higher harmonic harmonic currents in the motor. For the anti-jamming measures at the output of the inverter, the following aspects must be noted:
a The output of the inverter is not allowed to be connected to the capacitor, so as to avoid a large peak charging (or discharging) current when the inverter tube is turned on (off), and damage the inverter tube;
b When the output filter consists of an LC circuit, the side of the filter that is connected to the capacitor must be connected to the motor side.
(3) Shielding interference sources is the most effective way to suppress interference. Usually, the inverter itself is shielded by an iron shell to prevent electromagnetic interference from leaking; the output line is preferably shielded by a steel pipe, especially when the inverter is controlled by an external signal, the signal line is required to be as short as possible (generally within 20 m), and the signal line It adopts double-core shielding and is completely separated from the main circuit line (AC380V) and control line (AC220V). It must not be placed in the same pipe or trunk. The surrounding electronic sensitive equipment lines also require shielding. For effective shielding, the shield must be reliably grounded.
(4) Correct grounding can not only effectively suppress external interference, but also reduce the interference of the device itself to the outside world. In the actual application system, because the system power supply neutral line (middle line), ground line (protective grounding, system grounding) is not divided, the control system shielding ground (control signal shielding ground and main circuit wire shielding ground) chaotic connection, greatly reduced System stability and reliability.
For the inverter, the correct grounding of the main circuit terminal PE (E, G) is an important means to improve the inverter's ability to suppress noise and reduce the interference of the inverter. Therefore, it must be taken seriously in practical applications. The cross-sectional area of ​​the grounding conductor of the inverter should generally be no less than 2.5mm2 and the length should be controlled within 20m. It is recommended that the grounding of the inverter be separated from the grounding points of other power equipment and not be grounded.
(5) Using a reactor
The lower frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the inverter account for a high proportion, in addition to possible interference In addition to the normal operation of other equipment, because they consume a large amount of reactive power, the power factor of the line is greatly reduced. Serializing the reactor in the input circuit is an effective way to suppress lower harmonic currents. There are two main types depending on the wiring location:
The AC reactor is connected in series between the power supply and the input side of the frequency converter. Its main functions are:
By suppressing the harmonic current, the power factor is increased to (0.75-0.85);
Attenuating the impact of the inrush current in the input circuit on the inverter;
Impair the effects of power supply voltage imbalance.
The DC reactor is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to weaken the higher harmonic components in the input current. However, it is more effective than AC reactor in improving the power factor, up to 0.95, and has the advantages of simple structure and small volume.
(6) Reasonable wiring
For the interference signal propagating by induction, it can be weakened by reasonable wiring. The specific methods are:
a The power and signal lines of the equipment should be away from the input and output lines of the inverter;
b The power and signal lines of other equipment should be avoided in parallel with the input and output lines of the inverter;
According to the basic principles of electromagnetics, electromagnetic interference must have three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-interference is the most basic and most important anti-interference measure. Generally, the anti-and anti-discharge is used to suppress interference. The general principle is to suppress and eliminate the interference source, cut off the coupling channel of the interference to the system, and reduce the system interference signal. Sensitivity. Specific measures can be isolated, filtered, shielded, grounded, etc. in the project. The following points are the main steps to resolve the on-site interference:
1 Adopt software anti-interference measures: Specifically, the carrier frequency of the inverter is lowered by the man-machine interface of the inverter, and the value is lowered to an appropriate range. If this method does not work, then only the following hardware anti-jamming measures can be taken.
2 Conduct proper grounding: Through the specific investigation on site, we can see that the grounding situation at the site is not ideal. The correct grounding can not only effectively suppress external interference, but also reduce the interference of the equipment itself to the outside world. It is the most effective measure to solve the inverter interference. Specifically, it is to do the following two points:
(a) The main circuit terminal PE (E, G) of the inverter must be grounded. This grounding can be shared with the motor of the inverter, but it cannot be shared with other equipment. The grounding pile must be ground separately and the connection should be made. The location should be as far away as possible from the grounding point of the weak electrical equipment. At the same time, the cross-sectional area of ​​the grounding conductor of the inverter should be no less than 4mm2, and the length should be controlled within 20m.
(b) In the grounding of other electromechanical equipment, the protective earthing and working grounding shall be separately provided with grounding poles and finally into the electrical grounding point of the power distribution cabinet. The shielding ground of the control signal and the shielding ground of the main circuit conductor should also be separately grounded and finally connected to the electrical grounding point of the power distribution cabinet.
Shielding sources of interference: Shielding sources of interference is a very effective way to suppress interference. Usually, the inverter itself is shielded by an iron casing to prevent electromagnetic interference from leaking, but the output line of the inverter is preferably shielded by a steel pipe, especially when an external signal (4~20 mA signal is output from the controller) is used to control the inverter. The control signal line is required to be as short as possible (generally within 20m), and shielded twisted pair must be used and completely separated from the main circuit line (AC380) and the control line (AC220V). In addition, shielded twisted pairs, especially pressure signals, are required for electronically sensitive equipment lines in the system. And all the signal lines in the system must not be placed in the same piping or trunking as the main circuit and control lines. For effective shielding, the shield must be reliably grounded.
4 ConclusionThrough the analysis of the source and propagation path of the interference in the application process of the frequency converter, the practical countermeasures to solve these problems are put forward. With the application of new technologies and new theories on the frequency converter, the EMC requirements of the frequency converter are emphasized, and the frequency conversion has become a frequency conversion. The problems that must be faced in the design and application of the speed control transmission system are also one of the keys to the application and promotion of the frequency converter. These problems with the frequency converter are expected to be solved by the function and compensation of the frequency converter itself. The requirements of the industrial site and the social environment for inverters are constantly improving, and the real "green" inverters that meet the actual needs will soon be available. We believe that the EMC problem of the inverter will be effectively solved.
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