Inverter failure and its treatment
Inverter failure and its treatment
In various industrial control systems, with the widespread use of power electronic devices such as inverters, the electromagnetic interference (EMI) of the system has become increasingly serious, and the corresponding anti-jamming design technology (ie, electromagnetic compatibility EMC) has become increasingly important. The interference of the frequency converter system can sometimes directly cause the hardware damage of the system, sometimes it can not damage the system hardware, but often causes the microprocessor system program to run out of control, resulting in control failure, resulting in equipment and production accidents. Therefore, how to improve the system's anti-jamming ability and reliability is an important content that can not be ignored in 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 frequency converter, we must first understand the source of the interference and the mode of transmission, and then take different measures against these interferences.
First, the source of inverter interference
The first is interference from the external grid. Harmonic interference in the power grid mainly disturbs the inverter through the power supply of the inverter. There are a large number of harmonic sources in the power grid such as various rectification equipment, AC-DC interchangeable equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment. These loads all cause the distortion of the voltage and current waveforms in the power grid, thus causing harmful interference to other equipment in the power grid. If the power supply of the inverter is disturbed by the interference from the polluted AC grid, the grid noise will interfere with the inverter through the grid power supply circuit. Power supply interference to the inverter is mainly (1) overvoltage, undervoltage, instantaneous power failure (2) surge, drop (3) spike voltage pulse (4) RF interference.
1. Interference from thyristor converter equipment to frequency converter
When there is a thyristor converter device with a large capacity in the power supply network, the thyristor is always turned on during part of each phase half cycle, which easily makes the network voltage appear notch and the waveform is seriously distorted. It may cause the rectification circuit on the input side of the inverter to be damaged due to the occurrence of a large reverse recovery voltage, resulting in breakdown of the input circuit and burning.
2. The interference of the power compensation capacitor to the inverter
The power sector has certain requirements on the power factor of the power-using unit. For this reason, many users have adopted a method of centralized capacitance compensation at the substation to increase the power factor. During the transient process of compensation capacitor input or output, the network voltage may have a very high peak. As a result, the rectifier diode of the inverter may break down due to the excessive reverse voltage.
Followed by the inverter's own external interference. The rectifier bridge of the inverter is a non-linear load to the grid. The harmonics generated by it will generate harmonic interference to other electronic and electrical equipment in the same grid. In addition, inverter inverters mostly use PWM technology. When operating in switch mode and performing high-speed switching, a large amount of coupling noise is generated. Therefore, the inverter is an electromagnetic interference source for other electronic and electrical devices in the system.
Inverter input and output currents contain many harmonic components. In addition to the lower harmonics that can constitute reactive power losses in power supplies, there are many harmonic components with high frequencies. They will spread their energy in various ways, forming interference signals to the frequency converter itself and other equipment.
(1) Waveform of the input current The inverter's input side is a diode rectification and capacitance 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 of the power supply voltage and appears as a discontinuous shock wave. It has a strong high harmonic content. Relevant 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 Most inverter inverter bridges use SPWM modulation, and their output voltage is a series of rectangular waveforms whose duty cycle is distributed by sine law; due to the inductive nature of the motor stator windings, The stator current is very close to a sine wave. However, harmonic components equal to the carrier frequency are still large.
Second, the spread of interference signals
The frequency converter can generate harmonics with large power. Because of the large power, it has strong interference with other equipments in the system. Its interference path is the same as the general electromagnetic interference path. It is mainly divided into conduction (ie, circuit coupling), electromagnetic radiation, and induction. coupling. Specifically: First, electromagnetic radiation is generated on the surrounding electronic and electrical equipment; secondly, electromagnetic noise is generated on the directly driven motor, which increases the iron and copper consumption of the motor; and it conducts interference to the power supply and transmits it to the other equipment of the system through the distribution network. Finally, the frequency converter induces inductive coupling to other adjacent lines and induces an interference voltage or current. In the same way, the interference signals in the system interfere with the normal operation of the inverter through the same route.
(1) The circuit coupling mode is spread through the power network. Because the input current is a non-sinusoidal wave, when the capacity of the inverter is large, the network voltage will be distorted, affecting the work of other equipment. At the same time, the conducted interference generated at the output end will greatly increase the copper loss and iron loss of the direct-drive motor. Affects the running characteristics of the motor. Obviously, this is the main transmission of the inverter input current interference signal
Broadcast method.
(2) Inductive coupling method When the input circuit or output circuit of the inverter is in close proximity with the circuit of other devices, the higher harmonic signals of the inverter will be coupled to other devices through induction. There are two ways to sense:
a, electromagnetic induction, this is the main way of current interference signal;
b. Electrostatic induction method, which is the main method of voltage interference signals.
(3) The aerial radiation method uses electromagnetic waves to radiate into the air. This is the main mode of transmission of high-frequency harmonic components.
Third, the anti-jamming measures of frequency control system
According to the basic principle of electromagnetics, electromagnetic interference (EMI) must have three elements: electromagnetic interference sources, electromagnetic interference paths, and systems sensitive to electromagnetic interference. To prevent interference, hardware anti-jamming and software anti-jamming can be used. Among them, hardware anti-jamming is the basic and most important anti-jamming measure of the application measure system. Generally, the anti-jamming measures are started from two aspects, anti-jamming and anti-tampering. The general principle is to suppress and eliminate the interference source and cut off the interference channel to the system. Reduce the sensitivity of system interference signals. Specific measures can be used in engineering isolation, filtering, shielding, grounding and other methods.
1. The so-called "isolation of interference" means that the interference source and the parts susceptible to interference are isolated from the circuit so that they do not have electrical connections. In the variable frequency drive system, an isolation transformer is usually used on the power line between the power supply and the amplifier circuit to avoid conduction interference. The power isolation transformer can use a noise isolation transformer.
2. The purpose of setting the filter in the system circuit is to suppress the interference signal from the inverter through the power line to conduct interference to the power supply from the motor. In order to reduce electromagnetic noise and losses, an output filter can be set on the output side of the inverter; to reduce interference with 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, you can set a power noise filter on the power line to avoid conducting interference. In the input and output circuits of the inverter, in addition to the above-mentioned lower harmonic components, there are many harmonic currents with high frequencies, which will propagate their energy in various ways to form interference with other devices. signal. Filters are the primary means for attenuating higher-frequency harmonic components. According to the use of different locations, can be divided into:
(1) There are usually two additional input filters:
a. The line filter is mainly composed of inductor coils. It attenuates higher harmonic currents by increasing the line's impedance at high frequencies.
b. The radiating filter is mainly composed of high-frequency capacitors. It will absorb high-frequency harmonic components with radiant energy.
(2) The output filter also consists of an inductor coil. It can effectively weaken the higher harmonic components in the output current. Not only play a role in anti-jamming, but also can reduce the additional torque caused by the higher harmonic harmonic current in the motor. For the anti-interference measures at the output of the inverter, the following must be noted:
a. The output of the frequency converter is not allowed to be connected to the capacitor so as to prevent the inverter from conducting (turning off) instantaneously, resulting in a large peak charge (or discharge) current and damaging the inverter;
b. When the output filter consists of an LC circuit, one side of the filter that connects to the capacitor must be connected to the motor side.
3, shielding interference sources is the most effective way to suppress interference. Normally, the inverter itself is shielded with a steel shell to prevent its electromagnetic interference from leaking. The output line is preferably shielded by steel pipes. Especially when the external signal is used to control the inverter, the signal line is required to be as short as possible (generally less than 20m), and the signal line is Double-core shielded and completely separated from the main circuit line (AC380V) and control line (AC220V), must not be placed in the same pipe or slot, and the surrounding 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 not only make the system effectively suppress external interference, but also reduce the interference of the device itself to the outside world. In the practical application system, the chaotic connection of the system neutral (midline), ground (protection grounding, system grounding) and control system shield ground (control signal shielding ground and main circuit wire shielding ground) is 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 not 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 equipments.
5, using reactors
The proportion of harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonics, etc.) with lower frequency in the input current of the inverter is very high, and they may interfere with In addition to the normal operation of other equipment, but also because they consume a lot of reactive power, so that the power factor of the line dropped. Series reactors in the input circuit are effective ways to suppress lower harmonic currents. According to the different wiring locations, there are mainly the following two:
(1) The reactor is connected in series between the power supply and the input side of the frequency converter. Its main functions are:
a. Increase the power factor to (0.75-0.85) by suppressing harmonic currents;
b, weaken the impact of the inrush current in the input circuit on the inverter;
c. Weakening the effect of power voltage imbalance.
(2) The DC reactor is connected in series between the rectifier bridge and the filter capacitor. Its single function is to weaken the higher harmonic components in the input current. However, it is more effective than the AC reactor in improving the power factor, and can reach 0.95, and has the advantages of simple structure and small size.
6, rational wiring
For the interference signal that is transmitted through induction, it can be weakened through reasonable wiring. The 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) The power lines and signal lines of other equipment should be avoided in parallel with the input and output lines of the inverter;
Fourth, the conclusion
Through the analysis of the sources and propagation paths 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 importance of the EMC requirements of the frequency converter has become the frequency conversion. The problems that must be faced in the design and application of variable speed drive systems are also one of the keys to the application and promotion of frequency converters. These problems with the inverter are expected to be solved by the function and compensation of the inverter itself. The requirements for frequency converters in the industrial field and in the social environment are constantly increasing. Real “green†frequency converters that meet actual needs will soon be available. We believe that the EMC problem of the inverter will be effectively solved.
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