The source of AC converter interference comes from interference from the external power 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 rectifier equipment, AC and DC exchange equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment.
Most of the inverters of the inverter use PWM technology, and when working in switching mode and high-speed switching, a large amount of coupling noise occurs. 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 higher harmonic components.
In addition to the lower harmonics that can constitute the reactive power loss of the power supply, there are many harmonic components with high frequencies. It will convey its own energy in various ways to form interference signals to the inverter itself and other equipment.
The waveform of the input current, the input side of the inverter is the diode rectification and capacitor filtering circuit. Obviously, only when the line voltage UL of the power supply is greater than the DC voltage UD at both ends of the capacitor, there is a charging current in the rectifier bridge. As a result, the charging current is always present around the amplitude of the supply voltage in the form of a discontinuous shock wave. It has a strong higher harmonic component. The relevant information indicates that the harmonic components of the 5th and 7th harmonics in the input current are the largest, and they are 80% and 70% of the 50HZ fundamental respectively.
The waveform of output voltage and currentThe inverter bridge of most inverters adopts SPWM modulation mode, and its output voltage is a series of rectangular shaped waves with duty cycle distributed according to sinusoidal law; Due to the inductive nature of the stator windings 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.
In addition, these loads cause waveform distortion of voltage and current in the power grid, thereby causing harmful interference to other equipment in the power grid. If the power supply of the inverter is disturbed from the polluted AC grid and is not treated, the grid noise will interfere with the inverter through the grid power supply circuit. The interference of the power supply to the inverter mainly includes overvoltage, undervoltage, instantaneous power drop, surge, drop peak voltage pulse, and radio frequency interference.
Interference of thyristor converter equipment to the inverter When there is a thyristor converter equipment with large capacity in the power supply network, because the thyristor is always turned on in the local time of each phase and a half cycle, it is easy to make the network voltage notch, and the waveform is seriously distorted. The rectifier circuit on the input side of the inverter may be damaged due to the occurrence of a large reverse recovery voltage, which may cause the input circuit to break down and burn out.
Power to make up for the interference of capacitance to the inverter The power sector has certain requirements for the power factor of the power consumption unit, for this reason, many users use the method of centralized capacitance compensation in the substation to improve the power factor. During the transient process of compensating for capacitor input or cut-out, the network voltage may show high peaks, and as a result, the rectifier diode of the inverter may be subjected to excessive reverse voltage breakdown.
The second is the interference of the inverter itself to the outside. The rectifier bridge of the inverter is a nonlinear load to the power grid, and the harmonics generated have harmonic interference to other electronic and electrical equipment in the same power grid.
The inverter includes a rectifier circuit and an inverter circuit, and the input alternating current is converted into DC voltage through the rectifier circuit and flat-wave circuit, and then the DC voltage is converted into pulse voltage of different widths (called pulse width modulation voltage, PWM) through the inverter. By using this PWM voltage to drive the motor, you can adjust the torque and speed of the motor.
This principle of operation results in the following three types of electromagnetic interference:
1. Harmonic interference
The rectifier circuit will produce harmonic current, which produces a voltage drop on the impedance of the power supply system, resulting in the distortion of the voltage wavepattern, which interferes with many electronic devices (because most electronic devices can only work under the condition of sine wave voltage), and the common voltage distortion is the flattening of the top of the sine wave. When the harmonic current is constant, the voltage distortion is more serious in the case of weak power supply, and the characteristic of this interference is that it will interfere with the equipment that uses the same power grid, and has nothing to do with the distance between the equipment and the inverter;
2. Radio frequency conduction emission interference
Because the load voltage is pulse-shaped, the inverter draws current from the power grid is also pulse-shaped, and this pulse current contains a large number of high-frequency components, forming radio frequency interference, which is characterized by interference to the equipment that uses the same power grid, and has nothing to do with the distance between the equipment and the inverter;
3. Radio frequency radiation interference
RF radiation interference comes from the input and output cables of the frequency converter. In the above-mentioned RF conduction emission interference situation, when there is a radio frequency interference current on the input and output cables of the inverter, because the cable is equivalent to an antenna, electromagnetic wave radiation will inevitably be generated, resulting in radiation interference. 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 radiated interference. Radiated interference is characterized by the phenomenon of interference becoming severe when other electronic devices are in close proximity to the frequency converter.
According to the basic principles of electromagnetism, there are three elements that must be present to form electromagnetic interference: the source of electromagnetic interference, the path of electromagnetic interference, and the system sensitive to electromagnetic interference. In order to prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-interference is the most basic and important anti-interference measures, generally from the anti-amplification and amplification two aspects to suppress interference, its general principle is to suppress and eliminate the source of interference, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system interference signal. Specific measures can be used in the project isolation, filtering, shielding, grounding and other methods.
Key steps to address on-site interference:
1. Adopt software anti-interference measures
Specifically, the carrier frequency of the inverter is lowered through the human-machine interface of the inverter to an appropriate range. If this method does not work, then the following hardware anti-interference measures can only be taken.
2. Carry out correct grounding
Through the specific investigation of the site, we can see that the grounding situation on the site is not ideal. The correct grounding can not only effectively suppress external interference in the system, but also reduce the interference of the equipment itself to the outside world, which is an effective measure to solve the interference of frequency converters. Specifically, it is to do the following:
(1) The main circuit terminal PE (E, G) of the inverter must be grounded, and the grounding can be shared with the motor of the inverter, but not with other equipment, and the grounding pile must be driven separately, and the grounding point should be as far away as possible from the grounding point of the weak current equipment. At the same time, the cross-sectional area of the inverter grounding wire should not be less than 4mm2, and the length should be controlled within 20m.
(2) In the ground wire of other mechanical and electrical equipment, the protective grounding and the working grounding should be separately set up with a grounding electrode, and then merged into the electrical grounding point of the distribution cabinet. The shielding ground of the control signal and the shielding ground of the main circuit wire should also be separately set up with a separate grounding electrode, and then integrated into the electrical grounding point of the distribution cabinet.
3. Shield the source of interference
Shielding the source of interference is a very effective way to suppress interference. Usually the inverter itself is shielded with an iron shell, which can prevent its electromagnetic interference from leaking, but the output line of the inverter is better to be shielded with a steel pipe, especially when controlling the inverter with an external signal (output 4-20mA signal from the controller), the control signal line is required to be as short as possible (generally within 20m), and the shielded twisted pair must be used, and completely separated from the main power line (AC380) and the control line (AC220V). In addition, shielded twisted pair cables are required for the circuits of the electronic sensitive equipment in the system, especially for pressure signals. And all signal lines in the system must not be placed in the same piping or trunking as the main electrical line and control line. For the shield to be effective, the shield must be reliably grounded.
4. Reasonable wiring
Here's how:
(1) The power line and signal line of the equipment should be as far away as possible from the input and output lines of the inverter.
(2) The power line and signal line of other equipment should be avoided parallel to the input and output lines of the inverter.
If the above methods do not work, then continue with the following methods:
5. Isolation of interference
The so-called interference isolation refers to isolating the interference source from the part that is susceptible to interference from the circuit, so that they do not have electrical contact. Usually between the power supply and the amplifier circuit such as the controller and the transmitter, an isolation transformer is used on the power line to avoid conduction interference, and the power isolation transformer can be used to apply a noise isolation transformer.
6. Set the filter in the system line
The function of the device filter is to suppress the interference signal from the inverter through the power line to the power supply and the motor. In order to reduce electromagnetic noise and loss, an output filter can be set on the output side of the inverter; In order 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 such as controllers and transmitters in the line, a power noise filter can be installed on the power line of the equipment to avoid conduction interference.
Filters can be divided into the following categories according to the different locations where they are used:
(1) Input filter
There are usually two kinds:
a. Line filter: It is mainly composed of inductance coils, which weakens the harmonic current with higher frequency by increasing the impedance of the line at high frequency.
b. Radiation filter: It is mainly composed of high-frequency capacitors, which will absorb harmonic components with high frequency points and radiant energy.
(2) The output filter is also composed of an inductance coil
It can effectively weaken the higher harmonic component in the output current. It not only plays an anti-interference role, but also weakens the additional torque caused by the harmonic current generated by the higher harmonics in the motor. For the anti-interference measures at the output end of the inverter, the following aspects must be noted:
a. The output end of the inverter is not allowed to access the capacitor, so as not to generate a large peak charge (or discharge) current at the moment of power tube conduction (shutdown) and damage the power tube;
b. When the output filter is composed of LC circuits, the side of the capacitor connected to the filter must be connected to the motor side.
7. Adopt reactor
In the input current of the inverter, the harmonic components with lower frequencies (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) account for a very high proportion, 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 is greatly reduced. Serial insertion of reactors into the input circuit is an effective way to suppress lower harmonic currents. Depending on the wiring position, there are two main types:
(1) AC reactor
It 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 impact of the inrush current in the input circuit on the inverter;
c. Weaken the influence of power supply voltage imbalance.
(2) DC reactor
It is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, that is, to weaken the higher harmonic components in the input current. However, it is more effective than AC reactors in improving the power factor, up to 0.95, and has the advantages of simple structure and small size.
Therefore, the anti-interference measures of the inverter mainly include the installation of alternating current reactors and filters in the inlet part of the inverter, the incoming and outgoing wires adopt shielded cables, and the shielding layer of all cables is grounded together with the protective ground of the reactor, filter, inverter and motor, and the grounding point is separated from other grounding points and keeps sufficient distance. At the same time, the signal cable and the power cable of the inverter should not be arranged in parallel.
In addition, in order to prevent the inverter from interfering with the signal and control loop, it is necessary to use a separate isolated power supply for the controller, instrument and industrial computer.
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