First, the source of the inverter interference
The 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 mainly (1) overvoltage, undervoltage, instantaneous power loss (2) surge, drop (3) spike voltage pulse (4) 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. The interference of the power compensation capacitor 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) Waveform of input current The input side of the inverter 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 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) The waveform of output voltage and current Most of the inverter bridges of the inverter adopt SPWM modulation mode, and the output voltage is a series of rectangular waveforms whose duty cycle is distributed according to sinusoidal law; due to the inductive nature of the stator winding of the motor, The current of the stator is very close to a sine wave. However, the harmonic component equal to the carrier frequency is still large.
Second, the way the interference signal is transmitted
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 mode 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 propagation mode of the inverter 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. Static induction mode, which is the main way of voltage interference signal.
(3) The airborne radiation method radiates into the air by electromagnetic waves, which is the main mode of propagation of harmonic components with high frequency.
Third, the anti-jamming countermeasures of the frequency conversion speed control system
According 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 part that is susceptible to interference are isolated from the circuit so that they do not have 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:
(1) There are usually two types of input filters:
a. The line filter is mainly composed of an inductor coil. It attenuates higher frequency harmonic currents by increasing the impedance of the line at high frequencies.
b. The radiation filter is mainly composed of a high frequency capacitor. It will absorb harmonic components with high frequency and radiant energy.
(2) The output filter is also composed of an inductor. 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 frequency converter is not allowed to access the capacitor, so as to avoid a large peak charging (or discharge) current when the inverter tube is turned on (off), damage the inverter tube;
b. When the output filter is composed 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. The proportion of harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) with low frequency in the input current of the inverter is very high. In addition to their ability to interfere with the normal operation of other devices, they also consume a large amount of reactive power, which greatly reduces the power factor of the line. 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:
(1) The reactor is connected in series between the power supply and the input side of the inverter. Its main functions are:
a, by suppressing the harmonic current, the power factor is increased to (0.75-0.85);
b. weaken the impact of the surge current in the input circuit on the inverter;
c. Attenuate the effects of power supply voltage imbalance.
(2) 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:
(1) The power and signal lines of the equipment should be away from the input and output lines of the inverter;
(2) The power 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 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.
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