Interference Problems and Countermeasures in Frequency Converter Application

In various industrial control systems, with the widespread use of power electronic devices such as frequency converters, the electromagnetic interference (EMI) of the system has become increasingly serious, and the corresponding anti-interference design technology (i.e. electromagnetic compatibility EMC) has become increasingly important. The interference of the frequency converter system can sometimes directly cause damage to the system hardware. Sometimes, although it cannot damage the system hardware, it often causes the system program of the microprocessor to run out of control, resulting in control failure, thereby causing equipment and production accidents. Therefore, how to improve the anti-interference ability and reliability of the system is an important content that cannot 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 it comes to the anti-interference problem of frequency converters, we must first understand the source and propagation mode of the interference, and then take different measures against these interferences.

1. Sources of inverter interference

The first is interference from the external power grid. 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 various rectifiers, AC/DC switching equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment. These loads cause waveform distortion of the 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 interfered by the polluted AC power grid and is not processed, the grid noise will interfere with the inverter through the power supply circuit of the grid. The interference of the power supply to the inverter mainly includes (1) overvoltage, undervoltage, instantaneous power failure (2) surge, drop (3) peak voltage pulse (4) radio frequency interference.

1. Interference of thyristor commutation equipment on inverter

When there are large-capacity thyristor commutation devices in the power supply network, since the thyristor is always turned on for part of the time in each half cycle, it is easy to cause a notch in the network voltage and serious waveform distortion. It is possible that the rectifier circuit on the input side of the inverter will be damaged due to a large reverse recovery voltage, resulting in the breakdown and burning of the input circuit.

2. Interference of power compensation capacitor on inverter

The power sector has certain requirements for the power factor of power users. For this reason, many users use centralized capacitor compensation in substations to improve the power factor. During the transient process of the compensation capacitor being switched in or out, the network voltage may have a very high peak value, which may cause the inverter's rectifier diode to break down due to excessive reverse voltage.

The second is the external interference of the inverter itself. The inverter's rectifier bridge is a nonlinear load for the power grid, and the harmonics it generates will cause harmonic interference to other electronic and electrical equipment in the same power grid. In addition, the inverter of the inverter mostly adopts PWM technology, and when working in the switching mode and switching at high speed, a large amount of coupling noise is generated. Therefore, the inverter is an electromagnetic interference source for 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-order harmonics that can cause reactive power loss, there are also many high-frequency harmonic components. 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 charging current in the rectifier bridge only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor. 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-order harmonic component. 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 inverters use SPWM modulation, and their output voltage is a series of rectangular waves with a duty cycle distributed according to the sine law; due to the inductance of the motor stator winding, the stator current is very close to a sine wave. However, the harmonic component equal to the carrier frequency is still relatively large.

2. Propagation Mode of Interference Signal

The inverter can generate high-power harmonics. Due to the high power, it has strong interference to other equipment in the system. Its interference path is consistent with the general electromagnetic interference path, which is mainly divided into conduction (i.e. circuit coupling), electromagnetic radiation, and inductive coupling. Specifically: first, it generates electromagnetic radiation to the surrounding electronic and electrical equipment; second, it generates electromagnetic noise to the directly driven motor, which increases the iron loss and copper loss of the motor; and conducts interference to the power supply, which is transmitted to other equipment in the system through the distribution network; finally, the inverter generates inductive coupling to other adjacent lines, inducing interference voltage or current. Similarly, the interference signal in the system interferes with the normal operation of the inverter through the same path.

(1) The circuit coupling mode is to propagate through the power supply network. Since the input current is a non-sinusoidal wave, when the capacity of the inverter is large, the network voltage will be distorted, affecting the operation of other equipment. At the same time, the conducted interference generated at the output end greatly increases the copper loss and iron loss of the directly driven motor, affecting the operating characteristics of the motor. Obviously, this is the main propagation mode of the inverter input current interference signal.

(2) Inductive coupling method: When the input circuit or output circuit of the inverter is very close to the circuit of other equipment, the high-order harmonic signal of the inverter will be coupled to the other equipment through induction. There are two ways of induction:

a. Electromagnetic induction, which is the main way for current to interfere with signals;

b. Electrostatic induction method, which is the main way of voltage interference signal.

(3) The air radiation method is to radiate into the air in the form of electromagnetic waves. This is the main propagation method of high-frequency harmonic components.

In various industrial control systems, with the widespread use of power electronic devices such as frequency converters, the electromagnetic interference (EMI) of the system has become increasingly serious, and the corresponding anti-interference design technology (i.e. electromagnetic compatibility EMC) has become increasingly important. The interference of the frequency converter system can sometimes directly cause damage to the system hardware. Sometimes, although it cannot damage the system hardware, it often causes the system program of the microprocessor to run out of control, resulting in control failure, thereby causing equipment and production accidents. Therefore, how to improve the anti-interference ability and reliability of the system is an important content that cannot 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 it comes to the anti-interference problem of frequency converters, we must first understand the source and propagation mode of the interference, and then take different measures against these interferences.

1. Sources of inverter interference

The first is interference from the external power grid. 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 various rectifiers, AC/DC switching equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment. These loads cause waveform distortion of the 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 interfered by the polluted AC power grid and is not processed, the grid noise will interfere with the inverter through the power supply circuit of the grid. The interference of the power supply to the inverter mainly includes (1) overvoltage, undervoltage, instantaneous power failure (2) surge, drop (3) peak voltage pulse (4) radio frequency interference.

1. Interference of thyristor commutation equipment on inverter

When there are large-capacity thyristor commutation devices in the power supply network, since the thyristor is always turned on for part of the time in each half cycle, it is easy to cause a notch in the network voltage and serious waveform distortion. It is possible that the rectifier circuit on the input side of the inverter will be damaged due to a large reverse recovery voltage, resulting in the breakdown and burning of the input circuit.

2. Interference of power compensation capacitor on inverter

The power sector has certain requirements for the power factor of power users. For this reason, many users use centralized capacitor compensation in substations to improve the power factor. During the transient process of the compensation capacitor being switched in or out, the network voltage may have a very high peak value, which may cause the inverter's rectifier diode to break down due to excessive reverse voltage.

The second is the external interference of the inverter itself. The inverter's rectifier bridge is a nonlinear load for the power grid, and the harmonics it generates will cause harmonic interference to other electronic and electrical equipment in the same power grid. In addition, the inverter of the inverter mostly adopts PWM technology, and when working in the switching mode and switching at high speed, a large amount of coupling noise is generated. Therefore, the inverter is an electromagnetic interference source for 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-order harmonics that can cause reactive power loss, there are also many high-frequency harmonic components. 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 charging current in the rectifier bridge only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor. 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-order harmonic component. 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 inverters use SPWM modulation, and their output voltage is a series of rectangular waves with a duty cycle distributed according to the sine law; due to the inductance of the motor stator winding, the stator current is very close to a sine wave. However, the harmonic component equal to the carrier frequency is still relatively large.

2. Propagation Mode of Interference Signal

The inverter can generate high-power harmonics. Due to the high power, it has strong interference to other equipment in the system. Its interference path is consistent with the general electromagnetic interference path, which is mainly divided into conduction (i.e. circuit coupling), electromagnetic radiation, and inductive coupling. Specifically: first, it generates electromagnetic radiation to the surrounding electronic and electrical equipment; second, it generates electromagnetic noise to the directly driven motor, which increases the iron loss and copper loss of the motor; and conducts interference to the power supply, which is transmitted to other equipment in the system through the distribution network; finally, the inverter generates inductive coupling to other adjacent lines, inducing interference voltage or current. Similarly, the interference signal in the system interferes with the normal operation of the inverter through the same path.

(1) The circuit coupling mode is to propagate through the power supply network. Since the input current is a non-sinusoidal wave, when the capacity of the inverter is large, the network voltage will be distorted, affecting the operation of other equipment. At the same time, the conducted interference generated at the output end greatly increases the copper loss and iron loss of the directly driven motor, affecting the operating characteristics of the motor. Obviously, this is the main propagation mode of the inverter input current interference signal.

(2) Inductive coupling method: When the input circuit or output circuit of the inverter is very close to the circuit of other equipment, the high-order harmonic signal of the inverter will be coupled to the other equipment through induction. There are two ways of induction:

a. Electromagnetic induction, which is the main way for current to interfere with signals;

b. Electrostatic induction method, which is the main way of voltage interference signal.

(3) The air radiation method is to radiate into the air in the form of electromagnetic waves. This is the main propagation method of high-frequency harmonic components.

3. Anti-interference measures for variable frequency speed regulation system

According to the basic principles of electromagnetics, 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 important anti-interference measure for the application system. Generally, interference is suppressed from the two aspects of resistance and prevention. The general principle is to suppress and eliminate the interference source, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system to interference signals. Specific measures in engineering can be isolation, filtering, shielding, grounding and other methods.

1. The so-called interference isolation refers to isolating the interference source and the part susceptible to interference from the circuit so that they do not have electrical contact. In the variable frequency speed drive system, an isolation transformer is usually used on the power line between the power supply and the amplifier circuit to avoid conducted interference. The power isolation transformer can also be a noise isolation transformer.

2. The purpose of setting a filter in the system line is to suppress the interference signal from the inverter through the power line to the power supply from the motor. 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 equipment in the line, a power noise filter can be set on the power line to avoid conducted interference. In the input and output circuits of the inverter, in addition to the above-mentioned lower harmonic components, there are many high-frequency harmonic currents, which will spread their energy in various ways and form interference signals to other equipment. The filter is the main means to weaken the higher-frequency harmonic components. According to the different locations of use, it can be divided into:

(1) There are usually two types of input filters:

a. Line filters are mainly composed of inductor coils. They weaken high-frequency harmonic currents by increasing the impedance of the line at high frequencies.

b. The radiation filter is mainly composed of high-frequency capacitors, which will absorb high-frequency harmonic components with radiation energy.

(2) The output filter is also composed of an inductor coil. It can effectively weaken the high-order harmonic components in the output current. It not only plays an anti-interference role, but also can weaken the additional torque caused by high-order harmonic currents in the motor. For the anti-interference measures at the output end of the inverter, the following aspects must be paid attention to:

a. The output end of the inverter is not allowed to be connected to a capacitor, so as to avoid generating a large peak charging (or discharging) current at the moment when the inverter is turned on (off), thereby damaging the inverter;

b. When the output filter is composed of an LC circuit, the side of the filter 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 with an iron shell to prevent electromagnetic interference leakage; the output line is best shielded with 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 20m), and the signal line is double-core shielded and completely separated from the main circuit line (AC380V) and the control line (AC220V). It must not be placed in the same piping or cable trough, and the surrounding electronic sensitive equipment lines are also required to be shielded. In order to make the shielding effective, the shielding cover must be reliably grounded.

4. Correct grounding can not only effectively suppress external interference, but also reduce the interference of the equipment itself to the outside world. In the actual application system, the stability and reliability of the system are greatly reduced due to the confusion of the system power supply neutral line (neutral line), ground line (protective grounding, system grounding) and control system shielding ground (control signal shielding ground and main circuit wire shielding ground).
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 inverter's interference. Therefore, it must be taken very seriously in practical applications. The cross-sectional area of ​​the inverter grounding wire 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 cannot be shared.

5. Use reactor

The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the inverter is very high. In addition to interfering with the normal operation of other equipment, they also consume a lot of reactive power, which greatly reduces the power factor of the line. Inserting a reactor in series in the input circuit is an effective way to suppress low-frequency harmonic currents. Depending on the wiring position, there are mainly two types:

(1) The reactor is connected in series between the power supply and the input side of the inverter. Its main functions are:

a. Improve the power factor to (0.75-0.85) by suppressing harmonic current;

b. Reduce the impact of surge current in the input circuit on the inverter;

c. Reduce the impact 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 high-order harmonic components in the input current. However, it is more effective than the AC reactor in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size.

6. Organize the wiring

Interference signals transmitted by induction can be weakened by reasonable wiring. The specific methods are:

(1) The power cord and signal cord of the equipment should be kept away from the input and output cords of the inverter;

(2) The power lines and signal lines of other equipment should not be parallel to the input and output lines of the inverter;

IV. Conclusion

Through the analysis of the source and propagation path of interference in the application of frequency converters, practical countermeasures to solve these problems are proposed. With the continuous application of new technologies and new theories in frequency converters, paying attention to the EMC requirements of frequency converters has become a problem that must be faced in the design and application of variable frequency speed regulation transmission systems, and it is also one of the keys to the application and promotion of frequency converters. These problems existing in frequency converters are expected to be solved through the functions and compensation of the frequency converters themselves. The requirements of industrial sites and social environments for frequency converters are constantly increasing, and truly "green" frequency converters that meet actual needs will soon be available. We believe that the EMC problems of frequency converters will be effectively solved.