In a variety of industrial control systems, with the widespread use of power electronic devices such as frequency converters, the system's electromagnetic interference (EMI) is increasingly serious, the corresponding anti-interference design techniques (i.e., electromagnetic compatibility EMC) has become increasingly important. Frequency converter system interference can sometimes directly cause hardware damage to the system, and sometimes can not damage the hardware of the system, but often make the microprocessor system program operation out of control, resulting in control failure, thus causing equipment and production accidents. Therefore, how to improve the system's anti-interference ability and reliability is an important part of the development and application of automation devices can not be ignored, but also one of the keys to the application and promotion of computer control technology. When it comes to the anti-interference problem of frequency converter, first of all, we should understand the source of interference, propagation mode, and then take different measures for these interferences.
First, the source of inverter interference
The first is the interference from the external power grid. Harmonic interference in the grid mainly through the inverter power supply interference inverter. There are a large number of harmonic sources in the power grid such as various rectifier equipment, AC and DC interchange equipment, electronic voltage adjustment equipment, non-linear loads and lighting equipment. These loads all make the voltage and current in the grid to produce waveform distortion, thus producing harmful interference to other equipment in the grid. Frequency converter power supply from the polluted AC power grid interference if not dealt with, grid noise will be through the grid power circuit interference frequency converter. Power supply interference to the frequency converter are (1) over-voltage, under-voltage, instantaneous power loss (2) surge, drop (3) spike voltage pulse (4) radio frequency interference.
1, thyristor converter equipment on the inverter interference
When there is a large capacity thyristor converter equipment in the power supply network, because the thyristor always conducts in part of each phase half-cycle, it is easy to make the network voltage notch and the waveform is seriously distorted. It makes it possible for the rectifier circuit on the input side of the inverter to be damaged due to the occurrence of a large reverse reversal voltage, which can lead to the breakdown of the input circuit and burn out.
2, power compensation capacitor interference with the frequency converter
Power sector of the power factor of the power unit has certain requirements, for this reason, many users are in the substation using centralized capacitor compensation method to improve the power factor. In the transient process of compensation capacitor input or cut out, the network voltage is likely to have a very high peak value, as a result of which the rectifier diode of the frequency converter may be subjected to excessive reverse voltage and breakdown.
Secondly, the inverter itself to the external interference. The rectifier bridge of the inverter is a nonlinear load on the grid, and the harmonics it generates produce harmonic interference to other electronic and electrical equipment on the same grid. In addition, the inverter of the frequency converter mostly adopts PWM technology, when working in switching mode and making high-speed switching, it generates a lot of coupling noise. Therefore, the inverter is a source of electromagnetic interference to other electronic and electrical equipment in the system.
The input and output currents of the inverter contain many high harmonics. In addition to the lower harmonics that can constitute the reactive power loss of the power supply, there are many harmonic components of very high frequency. They will spread their energy in various ways, forming interference signals to the inverter itself and other equipment.
(1) Input current waveform The input side of the inverter is a diode rectifier and capacitor filter circuit. Obviously there is a charging current in the rectifier bridge only when the line voltage UL of the power supply is greater than the DC voltage UD at the ends of the capacitor. Therefore, the charging current always appears near the amplitude value of the supply voltage in the form of a discontinuous shock wave. It has a strong high harmonic component. The data show that the harmonic components of the 5th and 7th harmonics in the input current are the largest, which are 80% and 70% of the 50HZ fundamental wave, respectively.
(2) Output voltage and current waveforms The vast majority of inverter inverter bridge are used SPWM modulation, the output voltage for the duty cycle according to the sinusoidal distribution of a series of rectangular-shaped wave; due to the inductive nature of the motor stator winding, the stator current is very close to the sinusoidal wave. However, the harmonic components equal to the carrier frequency are still large.
Second, the propagation of the interference signal
Frequency converter can produce larger power harmonics, due to the larger power, other equipment on the system interference is strong, its interference path and general electromagnetic interference path is consistent, mainly divided into conduction (i.e. circuit coupling), electromagnetic radiation, inductive coupling. Specifically: first of all, the surrounding electronic and electrical equipment to produce electromagnetic radiation; secondly, the direct-drive motor to produce electromagnetic noise, making the motor iron and copper consumption increases; and conduction interference to the power supply, through the distribution network to other equipment in the system; and finally inverter to the neighboring other lines to produce inductive coupling, induction of the interference voltage or current. Similarly, the interference signal in the system through the same way to interfere with the normal operation of the inverter.
(1) Circuit coupling method, that is, through the power supply network propagation. As the input current is non-sinusoidal, when the capacity of the inverter is large, it will make the network voltage distortion, affecting the work of other equipment, and at the same time, the output side of the conduction interference generated by the direct drive of the motor copper loss, iron loss increased significantly, affecting the operating characteristics of the motor. Obviously, this is the main propagation mode of the inverter input current interference signal.
(2) Inductive coupling When the input circuit or output circuit of the inverter is close to the circuit of other equipment, the high harmonic signal of the inverter will be coupled to other equipment by induction. There are two ways of induction:
a, electromagnetic induction way, which is the main way of current interference signal;
b, electrostatic induction way, which is the main way of voltage interference signal.
(3) Airborne radiation, i.e., electromagnetic radiation into the air, which is the main propagation mode of high-frequency harmonic components.
Third, frequency control system anti-interference countermeasures
According to the basic principles of electromagnetism, the formation of electromagnetic interference (EMI) must have three elements: electromagnetic interference sources, electromagnetic interference pathways, electromagnetic interference sensitive systems. To prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-jamming is the application of the most basic and important anti-jamming measures system, generally from the resistance and prevention of the two aspects to inhibit interference, the general principle is to inhibit and eliminate the source of interference, cut off the interference to the system coupled channel, reduce the sensitivity of the system interference signal. Specific measures can be used in engineering isolation, filtering, shielding, grounding and other methods.
1, the so-called interference isolation, refers to the interference source from the circuit and susceptible to interference part of the isolation, so that they do not occur electrical connection. In the frequency control drive system, usually between the power supply and amplifier circuits on the power line using isolation transformers to avoid the conduction of interference, power supply isolation transformer can be applied to the noise isolation transformer.
2, in the system line to set the role of the filter 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, the output side of the frequency converter can be set up output filter; in order to reduce the power supply interference, the input side of the frequency converter can be set up input filter. If there is sensitive electronic equipment in the line, a power supply noise filter can be set on the power line to avoid conduction 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 spread their energy in various ways to form interference signals to other equipment. Filters are the main means used to attenuate the higher frequency harmonic components. Depending on where they are used, they can be categorized as.
(1) Input filters There are usually two types of filters:
a. Line filters are mainly composed of inductive coils. It attenuates higher frequency harmonic currents by increasing the impedance of the line at high frequencies.
b. Radiation filters are mainly composed of high-frequency capacitors. It will absorb the harmonic components of high frequency with radiant energy.
(2) Output filter It also consists of an inductive coil. It can effectively weaken the high harmonic components in the output current. It not only plays the role of anti-interference, but also weakens the additional torque caused by the high harmonic harmonic current in the motor. For the anti-interference measures of the output of the frequency converter, the following aspects must be paid attention to:
a. Capacitors are not allowed to be connected to the output side of the frequency converter, so as not to produce a large peak charging (or discharging) current at the moment of inverter tube conduction (shutdown), which may damage the inverter tube;
b. When the output filter is composed of LC circuit, the side of the access capacitor in the filter must be connected with the motor side.
3, shielding the source of interference is the most effective way to suppress interference. Usually the frequency converter itself with iron shielding, to prevent leakage of electromagnetic interference; output line is best shielded with steel pipe, especially when controlling the frequency converter with external signals, the signal line is required to be as short as possible (generally within 20m), and the signal line using a double-core shielding, and with the main power line (AC380V) and the control line (AC220V) is completely separated from the same piping or line trough, never put around the electronic Sensitive equipment lines also require shielding. In order to make the shielding effective, the shield must be reliably grounded.
4, the correct grounding can make the system effectively inhibit foreign interference, but also to reduce the equipment itself to the outside world interference. In the actual application of the system, due to the system power supply zero line (center line), ground (protective grounding, system grounding) is not divided, the control system shielding ground (control signal shielding ground and the main circuit conductor shielding ground) of the chaotic connection, greatly reducing the stability and reliability of the system.
For inverters, the correct grounding of the main circuit terminal PE (E, G) is an important means of improving the inverter's ability to suppress noise and reduce inverter interference, so it must be taken very seriously in practical applications. The cross-sectional area of the inverter grounding conductor should generally be not less than 2.5mm2, and the length should be controlled within 20m. It is recommended that the grounding of the inverter and other power equipment grounding separate, can not be common ground.
5, the use of reactor
In the input current of the frequency converter in the lower frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) accounted for a very high proportion of them in addition to possible interference with the normal operation of other equipment, but also because they consume a large amount of reactive power, so that the line of the power factor is greatly reduced. Stringing a reactor within the input circuit is an effective way to suppress lower harmonic currents. Depending on the wiring position, there are two main types:
(1) Reactor Connected in series between the power supply and the input side of the inverter. Its main functions are:
a. Increase the power factor to (0.75-0.85) by suppressing the harmonic current;
b. Weaken the inrush current in the input circuit to the inverter;
c, weaken the impact of power supply voltage imbalance.
(2) DC reactor connected in series between the rectifier bridge and filter capacitor. It has a single function, which is to weaken the high harmonic components in the input current. However, it is more effective than AC reactor in improving power factor, which can reach 0.95, and has the advantages of simple structure and small size.
6, rational wiring
For the interference signal propagated through the induction method, can be weakened by the way of rational wiring. Specific methods are:
(1) The power line and signal line of the equipment should be far away from the input and output lines of the inverter;
(2) Other equipment power lines and signal lines should be avoided and inverter input and output lines parallel;
Fourth, the conclusion
Through the analysis of the sources and propagation paths of interference in the process of frequency converter application, practical countermeasures to solve these problems have been put forward. With the continuous application of new technologies and new theories on frequency converters, attention to the EMC requirements of frequency converters has become a problem that must be faced by the design and application of frequency converter speed control drive systems, and it is also one of the keys to frequency converter application and promotion. These problems existing in the frequency converter are expected to be solved by the function and compensation of the frequency converter itself. Industrial site and the social environment of the frequency converter requirements continue to improve, to meet the actual needs of the real "green" frequency converter will soon come out. We believe that the EMC problem of inverter will be solved effectively.