In a variety of industrial control systems, with the widespread use of power electronic devices such as frequency converters, the electromagnetic interference (EMI) of the system is becoming more and more serious. Corresponding anti-interference design techniques (ie EMC) have become more and more important. Inverter system interference can sometimes directly cause system hardware damage, sometimes though not damage the system hardware, but often make the microprocessor system program run 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 that can not be ignored in the development and application of automation devices and is also one of the keys to the application and popularization of computer control technology. Speaking of anti-interference inverter, we must first understand the source of interference, transmission, and then take different measures for these interference.
Inverter trouble and processing method
First, the source of inverter interference
The first is interference from the external power grid. The harmonic interference in the power grid mainly interferes with the inverter through the power supply of the inverter. There are a large number of harmonic sources in the power grid such as all kinds of rectification equipment, AC-DC exchange equipment, electronic voltage regulation equipment, nonlinear loads and lighting equipment. These loads cause waveform distortion in the voltage and current in the power grid, which may cause harmful interference to other devices in the power grid. If the power supply of the inverter receives interference from the AC power grid which is polluted, the noise of the power grid will interfere with the inverter through the power supply circuit of the power grid. Power supply interference on the inverter are (1) overvoltage, undervoltage, instantaneous power-down (2) surge, drop (3) peak voltage pulse (4) radio frequency interference.
1, thyristor converter equipment on the inverter interference
When there is a thyristor commutation device with larger capacity in the power supply network, the thyristor always conducts during part of the half-cycle of each phase, which tends to make the network voltage notched and the waveform seriously distorted. It makes it possible for the rectifier circuit on the input side of the inverter to be damaged due to the large reverse recovery voltage, resulting in breakdown of the input circuit and burning.
2, the power compensation capacitor on the inverter interference
Power sector of the power factor of the unit has certain requirements, for which many users are in the substation use of concentrated capacitance compensation method to improve the power factor. During the transient process of compensating capacitor input or cut-off, the network voltage may have a high peak value. As a result, the rectifier diode of the frequency converter may break down due to the excessive reverse voltage.
Followed by the inverter itself on the external interference. Rectifier bridge inverter is a nonlinear load on the power grid, which produces harmonics on the same power grid of other electronic and electrical equipment harmonic interference. Inverter inverter inverter most of the use of PWM technology, when working in the switch mode and make high-speed switching, resulting in a large number of coupling noise. Therefore, the inverter is an electromagnetic interference source to other electronic and electrical equipment in the system.
Inverter input and output current, contains a lot of higher harmonic components. In addition to the lower harmonics that make up the reactive power loss of the power supply, there are many harmonic components at high frequencies. They will be in a variety of ways to spread their energy out of the formation of the inverter itself and other devices interfere with the signal.
(1) Input Current Waveform The input side of the inverter is a diode rectifier and capacitor filter circuit. Obviously, only the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor, the charging current is present in the rectifier bridge. Therefore, the charging current always appears near the amplitude of the power supply voltage, in the form of discrete shock waves. It has a strong harmonic component. Relevant data show that the harmonic components of the 5th harmonic and the 7th harmonic in the input current are the largest, which are respectively 80% and 70% of the 50HZ fundamental wave.
(2) Output Voltage and Current Waveform Most inverter inverter bridges adopt SPWM modulation mode. The output voltage is a series of rectangular shaped waveforms whose duty cycle is distributed in a sinusoidal law. Due to the nature of the inductance of the stator winding of the motor, The stator current is very close to the sine wave. However, the harmonic components which are equal to the carrier frequency are still larger.
Second, the way the signal interference
Inverters can generate larger power harmonics. Because of their large power, they have strong interference with other devices in the system. Their interference paths are consistent with those of common electromagnetic interference. They are mainly divided into conduction (ie, circuit coupling), electromagnetic radiation coupling. The method includes the following steps: firstly, generating electromagnetic radiation to the surrounding electronic and electrical equipment; secondly, generating electromagnetic noises to the directly driven motor so as to increase the iron consumption and the copper consumption of the motor; conducting the interference to the power supply to other equipment of the system through the power distribution network; Finally, the inverter generates inductive coupling to other adjacent lines and induces interference voltage or current. Similarly, the interference signal in the system interferes with the normal operation of the frequency converter in the same way.
(1) circuit coupling means that through the power network transmission. As the input current is non-sinusoidal, when the capacity of the inverter is large, the network voltage will be distorted, which will affect the work of other equipment workers. At the same time, the conducted interference generated at the output causes the copper loss of the directly driven motor and greatly increases the iron loss. Affect the operating characteristics of the motor. Obviously, this is the main input of the inverter input current interference signal
(2) Inductive coupling When the inverter input circuit or output circuit and other equipment circuit to get close, the inverter harmonic signal will be inductively coupled to other devices to go. There are two ways to sense:
a, electromagnetic induction, which is the main way of current interference signal;
b, electrostatic induction, which is the main way voltage interference signal.
(3) Aerial radiation means radiating into the air by means of electromagnetic waves, which is the main mode of transmission of harmonic components with high frequency.
Third, the frequency control system anti-jamming measures
According to the basic principles of electromagnetic, the formation of electromagnetic interference (EMI) must have three elements: sources of electromagnetic interference, electromagnetic interference, electromagnetic interference-sensitive systems. To prevent interference, you can use hardware anti-jamming and software anti-jamming. Among them, the hardware anti-jamming is the most basic and most important anti-jamming measures of the applied measure system. Generally speaking, the anti-jamming is suppressed by both anti-and anti-jamming. The general principle is to suppress and eliminate the interference source, cut off the interfering 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 interference isolation, refers to the circuit from the interference source and vulnerable parts isolated from, so that they do not have electricity contact. In the variable frequency drive system, usually the isolation transformer is 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 to the noise isolation transformer.
2, set the filter in the system line role is to suppress interference signals from the inverter through the power line conduction interference to the power from the motor. To reduce electromagnetic noise and losses, an output filter can be set on the output side of the inverter and an input filter on the input side of the inverter to reduce interference to the power supply. If there are sensitive electronic devices in the circuit, you can set the power supply noise filter on the power line to avoid conducted interference. In the input and output circuits of the frequency converter, in addition to the above lower harmonic components, there are many high frequency harmonic currents that will propagate their energy in various ways to create interference to other devices signal. The filter is the main means used to attenuate higher frequency harmonic components. According to the use of different locations, can be divided into:
(1) The input filter usually has two kinds:
a, the line filter is mainly composed of inductor coil. It weakens the higher harmonic currents by increasing the impedance of the line at high frequencies.
b, the radiation filter is mainly composed of high-frequency capacitors. It will absorb the high frequency, harmonic components with radiant energy.
(2) The output filter is also composed of an inductor. It effectively attenuates the higher harmonic components of the output current. Not only play a role in anti-jamming, and can weaken the motor by the harmonic current caused by harmonic currents of additional torque. For the anti-interference measures at the output of the inverter, the following must be observed:
a, the output of the frequency converter is not allowed to access the capacitor, so as not to turn on (turn off) the inverter tube momentarily, resulting in a large peak charging (or discharging) current damage to the inverter tube;
b. When the output filter consists of an LC circuit, one side of the capacitor that is connected to the capacitor must be connected to the motor side.
3, shielding the source of interference is the most effective way to suppress interference. Generally, the inverter itself is shielded with iron shell to prevent its electromagnetic interference from leaking. The output wire is preferably shielded with steel pipe. In particular, when controlling the inverter with external signals, the signal cable should be as short as possible (usually within 20m) and the signal cable Use double-core shield, and completely separate from the main circuit line (AC380V) and control line (AC220V), must not be placed in the same pipe or slot, the surrounding electronic-sensitive equipment lines also require shielding. To make the shield effective, the shield must be grounded.
4, the correct grounding can both 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 control system shielding ground (the control signal shielding ground and the main circuit wire shielding ground) is greatly reduced due to the zero line (neutral line) of the system power supply, the ground line (protective earth and system ground) System stability and reliability.
For the inverter, the correct grounding of the main circuit terminals PE (E, G) is an important means to improve the inverter’s ability of suppressing noise and reducing the interference of the inverter, so it must be taken seriously in practical application. Inverter grounding wire cross-sectional area should generally not be less than 2.5mm2, length control within 20m. It is recommended that the grounding of the inverter be separated from the grounding points of other power equipment and that it can not be common ground.
5, using the reactor
Among the input current of the inverter, the proportion of the harmonic components with the lower frequencies (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) is very high, except for their possible interference In addition to the normal operation of other equipment, but also because they consume a large amount of reactive power, the power factor of the line is greatly reduced. Chopping a reactor in the input circuit is an effective way to suppress lower harmonic currents. Depending on the location of the wiring, there are two main types:
(1) The reactor is 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 input circuit inrush current impact on the inverter;
c, weaken the impact of power supply 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 of the input current. But in improving the power factor than the AC reactor effective, up to 0.95, and has the advantages of simple structure, small size.
6, reasonable wiring
For the interference signal transmitted by sensing, it can be weakened by means of reasonable wiring. Specific methods are:
(1) The power cord and signal cable of the equipment should be far away from the input and output wires of the inverter;
(2) The power lines and signal lines of other equipment should be avoided from being parallel with the input and output lines of the inverter;
Fourth, the conclusion
Through the analysis of the source and route of the interference in the process of application of the inverter, the practical countermeasures to solve these problems are put forward. With the application of new technologies and new theories in the inverter, the EMC requirement of the inverter has been paid attention to. Speed control system design, applications must face the problem, but also the drive application and promotion of one of the keys. These problems existing in the inverter are expected to be solved through the functions and compensation of the inverter itself. Industrial site and social environment of the inverter requirements continue to meet the actual needs of the real “green” inverter will soon be available. We believe the inverter EMC problems will be effectively solved.