Abstract : This paper analyzes the interference formation mechanism of frequency converters and its harm to electrical systems, and proposes common methods to suppress such interference. The results show that harmonics and electromagnetic radiation are the main interference factors generated by frequency converters. By adopting suppression, isolation, filtering, shielding and grounding methods in engineering, these interference factors can be effectively suppressed, thereby ensuring the normal use of electrical systems.
Keywords : frequency converter; interference, harmonics; suppression
introduction
With the development of power electronics technology, power supply systems have seen a significant increase in nonlinear loads, particularly frequency converters. These converters are widely used, from low-voltage, small-capacity household appliances to high-voltage, large-capacity industrial inverters, leading to increasingly severe electromagnetic interference (EMI). Consequently, corresponding anti-interference design (i.e., electromagnetic compatibility, EMC) has become increasingly important. Sometimes EMI can directly damage system hardware; other times, while not damaging hardware, it can cause microprocessor programs to malfunction, resulting in control failures and ultimately, equipment accidents. Therefore, improving the anti-interference capability and reliability of systems is a crucial aspect of the development and application of automation devices, and a key factor in the application and promotion of computer control technology.
I. Harms of Harmonics and Electromagnetic Radiation to Power Grids and Other Systems
- Harmonics cause additional harmonic losses in electrical components in the power grid, reducing the efficiency of power transmission, distribution, and power consumption equipment.
- Harmonics can be conducted through the power grid to other electrical appliances, affecting their normal operation. For example, harmonics can cause mechanical vibrations in transformers, leading to localized overheating, insulation aging, shortened lifespan, and even damage. Furthermore, conducted harmonics can interfere with the normal operation of the internal software or hardware of electrical equipment.
- Harmonics can cause localized series or parallel resonances in the power grid, thereby amplifying the harmonics.
- Harmonic or electromagnetic interference can cause relay protection settings to malfunction, making electrical instruments inaccurate or even unable to function properly.
- Electromagnetic interference can disrupt weak electrical signals such as control and detection signals passing near the output conductors of a frequency converter. In severe cases, this can prevent the system from obtaining accurate detection signals or cause control system malfunctions. Generally, the impact of frequency converters on systems with large grid capacity is not very noticeable, which is why harmonics are not taken seriously by most users. However, for systems with small grid capacity, the interference caused by harmonics cannot be ignored.
II. Mechanism of Harmonic Generation
The main circuit of a frequency converter is generally composed of AC-DC-AC. An external 380V/ 50Hz power supply is uncontrolled rectified into DC voltage by a three-phase bridge circuit, then filtered by a filter capacitor and inverted into a variable-frequency AC signal by high-power transistor switching elements. In the rectifier circuit, the input current waveform is an irregular rectangular wave, which can be decomposed into a fundamental wave and various harmonics according to Fourier series. The higher harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by an SPWM (Pulse Width Modulation) carrier signal. For high-power GTR (Power Transistor) inverter elements, the SPWM carrier frequency is 2-3kHz , while for high-power IGBT (Insulated Gate Bipolar Transistor) inverter elements, the maximum SPWM carrier frequency can reach 15kHz . Similarly, the output circuit current signal can also be decomposed into a fundamental wave containing only a sine wave and other harmonics, with higher harmonic currents directly interfering with the load. Furthermore, higher harmonic currents also radiate into space through cables, interfering with nearby electrical equipment.
III. Main Electromagnetic Interference Sources and Pathways of Variable Frequency Speed Control Systems
- Frequency converters can generate high-power harmonics. Due to their high power, they can cause strong interference to other equipment in the system. The interference paths are the same as those of general electromagnetic interference, mainly consisting of conduction (i.e., circuit coupling), electromagnetic radiation, and inductive coupling.
- Circuit coupling refers to the propagation of interference signals through the power network. Since the input current is non-sinusoidal, a large input current will distort the network voltage, affecting the operation of other equipment. Simultaneously, conducted interference at the output significantly increases the copper and iron losses of the directly driven motor, affecting its operating characteristics. Clearly, this is the primary propagation method of inverter input current interference signals.
- Inductive coupling occurs when the inverter's input or output circuit is very close to the circuits of other devices, causing high-order harmonic signals to couple into those devices via induction. There are two types of induction coupling:
a) Electromagnetic induction, which is the main way current interferes with signals;
b) Electrostatic induction, which is the main way voltage interference signals are transmitted.
Airborne radiation refers to the radiation of electromagnetic waves into the air, which is the main propagation mode of high-frequency harmonic components.
IV. Commonly Used Methods for Suppressing Harmonic Interference
Harmonics propagate through conduction and radiation. Solving conducted interference mainly involves filtering out or isolating the high-frequency current in the circuit. Solving radiated interference involves shielding the radiation source or the affected line. In engineering, specific measures include suppression, isolation, filtering, shielding, and grounding.
1. Inhibition
a) Add AC/DC reactors
Installing a reactor essentially increases the internal impedance of the inverter's power supply from the outside. Installing a reactor on the AC or DC side of the inverter, or both simultaneously, can suppress harmonic currents. With AC/DC reactors (see Figure 1), the THDV (voltage distortion rate) of the incoming line current is reduced by 30%–50%, which is about half the harmonic current without a reactor.
b) Multiphase pulse rectification
When conditions permit or the required harmonic distortion is to be kept relatively low, multiphase rectification can be used. A 12-phase pulse rectification achieves a THD V of 10%–15%; an 18-phase pulse rectification achieves a THD V of 3%–8%, meeting the requirements of EN 61000-3-12 and IEEE 519-1992 standards. The disadvantages are the need for dedicated transformers and rectifiers, which hinders equipment modification and results in higher costs.
2. Isolation
Interference isolation refers to isolating the interference source from the susceptible part of the circuit, preventing them from being electrically connected. In variable frequency speed control transmission systems, isolation transformers (as shown in Figure 2) are typically used on the power line between the power supply and amplifier circuit to prevent conducted interference. Noise isolation transformers can be used as power isolation transformers. Optocouplers (as shown in Figure 3) are also commonly used to achieve electrical-to-optical-to-electrical isolation. They effectively prevent the entry of interference sources, reliably isolate signals, and are easily configured into various functional states.
3. Filtering
a) The purpose of setting up filters is to suppress interference signals that are conducted from the frequency converter through the power lines to the power supply and motor. To reduce power supply noise and losses, an output filter can be installed on the output side of the frequency converter. To reduce interference to the power supply, an input filter can be installed on the input side of the frequency converter. If there are sensitive electronic devices in the circuit, a power supply noise filter can be installed on the power line to prevent conducted interference.
b) The basic working principle of an active power filter is that it injects a compensation current into the power grid with the same amplitude but opposite phase to the load harmonic current, thereby enabling the fundamental component of the power grid current to track and compensate for harmonics with varying frequency and amplitude, thus canceling out the harmonic current in the power grid, and therefore has received widespread attention.
In 1976, L.G. Y.G. and E.C. Styaula proposed using PWM inverters to construct active power filters. These active power filter circuits using PWM inverters have now become the basic structure of active power filters. Voltage-source inverters control the output voltage as required, providing accurate current to the grid. Current-source inverters modulate direct current (DC) into an AC pulse train, which is then demodulated into an accurate current by a filter on the AC output side. Current-source inverters require the DC current to be matched with the maximum compensation current. The disadvantages of current-source inverters are high losses and the need for a demodulator filter; therefore, voltage-source inverters are typically used instead of current-source inverters.
The schematic diagrams of the two filters mentioned above are shown in Figures 4 and 5.
The two types of active power filters mentioned above have the following advantages:
① As a source of high-order harmonic current, it is not affected by system impedance.
② There is no resonance phenomenon, and changes in the system structure will not affect the compensation effect.
③ In principle, it is superior to LC filters, as a single device can compensate for all harmonics.
④ It can accurately compensate for changes in the frequency of higher harmonics.
⑤ Since the device itself can perform output limiting, even high-order harmonic compensation is a great advantage.
However, active filters also have the following drawbacks:
⑥ The control is complex and the cost is high.
⑦ It consumes a lot of energy.
4. Shielding
Shielding the source of interference is the most effective way to suppress interference. Typically, the frequency converter itself is shielded with a metal casing to prevent electromagnetic interference leakage. Output lines should ideally be shielded with steel conduit, especially when the frequency converter is controlled by external signals. Signal cables should be as short as possible (generally within 20m), and double-core shielded and double-cross-shielded cables should be used, completely shielded from the main circuit and control loop. For effective shielding, the shielding enclosure must be reliably grounded.
5. Grounding
Practice has proven that grounding is often an important means of suppressing noise and preventing interference. A good grounding method can greatly suppress the coupling of internal noise, prevent the intrusion of external interference, and improve the system's anti-interference capability. Inverters can be grounded in several ways, including multi-point grounding, single-point grounding, and grounding via the busbar. The appropriate method should be chosen based on the specific circumstances, and care should be taken to avoid causing interference to the equipment due to poor grounding.
Single-point grounding refers to a circuit or device where only one physical point is defined as the grounding point. It performs well at low frequencies. Multi-point grounding means that each grounding point in the device is directly connected to the nearest grounding point. It performs well at high frequencies. Hybrid grounding uses a combination of single-point and multi-point grounding based on the signal frequency and grounding wire length. The frequency converter itself has a dedicated grounding terminal (PE), which must be grounded for safety and noise reduction. The grounding wire cannot be connected to the outer casing of the electrical equipment, nor can it be connected to the neutral wire. A thicker, shorter wire can be used, with one end connected to the PE terminal and the other end connected to the grounding electrode. The grounding resistance should be less than 100Ω, the grounding wire length should be within 20m, and the location of the grounding electrode should be carefully selected.
The above anti-interference measures can be selected and used reasonably according to the anti-interference requirements of the system, as shown in Figure 6:
V. Other issues to consider in the design of electrical control systems
1. Ensure all equipment in the control cabinet is properly grounded. Additionally, control devices connected to the frequency converter (such as PLCs or PID controllers) must share a common ground.
2. When installing and wiring, separate the power cord and control cable, for example, by using a separate cable tray. If the control circuit connection wires must cross the power cable, they should be wired at a 90° angle.
3. When using shielded wires or twisted-pair cables to connect control circuits, ensure that the unshielded portion is as short as possible.
4. Ensure that the contactors in the control cabinet have arc-extinguishing capabilities. AC contactors should use R - C suppressors, or varistors. This is especially important if the contactors are controlled by relays from the frequency converter.
5. When armored cables are used as motor wiring, the shielding layer must be grounded.
6. When arranging equipment, weak current control equipment that is easily affected by interference should be separated from the frequency converter as much as possible. For example, the power distribution cabinet should be placed between the frequency converter and the control equipment.
7. Minimize unnecessary wiring between the frequency converter and the control system to avoid conducted interference. Except for the necessary control lines between the control system and the frequency converter, other wiring, such as control power supplies, should be kept separate. Since both the control system and the frequency converter require 24V DC power, manufacturers often use a single DC power supply split into two separate lines to power the two systems in order to save on the cost of a separate power supply. Sometimes, the frequency converter can cause conducted interference to the control system through the DC power supply. Therefore, it should be specifically specified in the design or ordering process that two separate DC power supplies should be used to power the two systems.
8. Take precautions to prevent resonance. Because the stator current contains high-order harmonic components and the motor torque contains pulsating components, resonance may occur between the motor's vibration and mechanical vibration, leading to equipment malfunction. The inherent resonant frequency of the load should be identified beforehand, and the frequency jump function of the inverter should be used to avoid the resonant frequency point.
VI. Examples of Harmonic Interference Suppression
Example 1:
In our factory's 3-wire screw pressure closed-loop system, after the frequency converter starts, the control system instrument (RKC, F900F810*8—AD instrument input 4-20mA) displays pressure fluctuating around ±2MPa, while the pressure probe (Dynisco 0-35MPa output 4-20mA) shows pressure fluctuation. The frequency on the frequency converter (Yaskawa 616H3) fluctuates around ±5H 2 (±2H 2 ). Because it is a closed-loop system, any fluctuation in one area will cause fluctuations throughout the entire pressure system, as shown in Figure 7.
The initial approach was to switch the instrument to manual control with a fixed output of 60%. However, the pressure still fluctuated. The current instrument output is 13.6 mA from the 36H2 frequency converter, ruling out the frequency converter and the instrument output. Adjusting the instrument's PID control (d↑ - small derivative coefficient, insufficient response to instantaneous changes, weak reaction; I↓ - excessive integral coefficient, causing the derivative response to be submerged and distorted) had little effect. Improper settings were ruled out. The analysis concluded that interference was the cause. Oscilloscope analysis revealed a high proportion of low-frequency harmonics (5th, 7th, 11th, and 13th harmonics) in the frequency converter's input current. These harmonics, besides potentially interfering with the normal operation of other equipment, also consume a large amount of reactive power, significantly reducing the power factor of the circuit.
To address frequency converter interference, necessary measures will be taken, starting with the following aspects:
1. Install an AC reactor
An AC reactor is connected in series between the power supply and the input side of the frequency converter. Its purpose is to improve the power factor to (0.75-0.85) by suppressing harmonic currents, reducing the impact of wasted current in the input circuit on the frequency converter, and reducing the impact of power supply voltage imbalance.
2. Install a DC reactor
A DC reactor, connected in series between the rectifier bridge and the filter capacitor, aims to further reduce the high-order harmonic components of the input current, but it is more effective than an AC reactor in improving the power factor by up to 0.95. (See Figure 13)
3. Install filters on the input and output sides.
Input filters are generally divided into two types:
- Line filters are mainly composed of inductors. They reduce high-frequency harmonic currents by increasing the line's impedance at high frequencies.
- Radiation filters are mainly composed of high-frequency capacitors, which absorb high-frequency harmonic components that radiate energy.
Output filters are generally also composed of inductors. They can effectively reduce high-order harmonic components in the output current. They not only provide anti-interference but also reduce the additional torque caused by high-order harmonic currents in the motor. The following aspects must be considered when implementing anti-interference measures at the inverter output:
1) Capacitors are not allowed to be connected to the output terminal of the frequency converter, so as to avoid generating a very high peak charging (or discharging) current at the moment when the inverter tube is turned off (ventilation shut-off), which may damage the inverter tube;
2) When the output filter is composed of an LC circuit, the side of the capacitor connected inside the filter must be connected to the motor side.
4. Inspect the grounding system
Because the neutral wire (electric wire) and ground wire (protective ground, system ground) of the system power supply are not distinguished, the chaotic connection of the shielding ground of the control system (control signal shielding ground and main circuit wire shielding ground) will greatly reduce the stability and reliability of the system.
After processing, the pressure fluctuation was within ±0.3MPa and the frequency fluctuation was within ± 1H2 . Harmonic interference was basically suppressed, and the system worked normally.
Example 2
Our factory has a VC443 drawing machine system with 144 temperature points controlled by a single-chip microcomputer and driven by a frequency converter. When the frequency converter is not working, the temperature display is within a normal ±1℃ range. However, when the frequency converter is working, the temperature fluctuates, with a cycle of ±15℃ every approximately 30 seconds. Analysis suggests this is caused by interference. Further investigation with on-site maintenance personnel revealed that one of the two 5V power supplies on the original control board had burned out, and a single switching power supply was then used. Afterward, the abnormal temperature occurred upon startup. The problem is suspected to be with the power supply.
The microcontroller uses opto-isolation in the interface circuit, as shown in Figure (2) above. The reason why the optocoupler can effectively suppress spikes and various noise interferences while transmitting signals, and greatly improve the signal-to-noise ratio of the channel, is mainly due to the following reasons:
1. The input impedance of the optocoupler is very small, only a few hundred ohms, while the impedance of the interference source is much larger, typically 10⁵ – 10⁶ Ω. Because there is insufficient energy, the diode cannot emit light and is thus suppressed.
2. There is no electrical connection or common ground between the input and output circuits of the optocoupler; the distributed capacitance between them is extremely small, while the insulation resistance is very large. Therefore, various interference noises on one side of the circuit are difficult to feed to the other side through the optocoupler, thus avoiding the generation of interference signals from common impedance coupling.
After careful analysis, it was found that the input and output sections of an optocoupler must use separate power supplies. If both ends share a single power supply, the isolation function of the optocoupler will be meaningless. When using an optocoupler to isolate input and output channels, all signals (including digital signals, control signals, and status signals) must be completely isolated, ensuring that there is no electrical connection between the isolated sides; otherwise, such isolation is meaningless. The temperature control functioned normally after the power supplies were separated.
in conclusion
Through analysis of the sources and propagation paths of interference during application, practical countermeasures for solving these problems are proposed. A large amount of electromagnetic interference, if not suppressed, will affect the normal operation of the entire control system. However, completely eliminating electromagnetic interference is unrealistic. Electromagnetic interference suppression should adopt appropriate suppression measures based on different components and different electromagnetic environments, with the normal operation of the system as the benchmark. It is unnecessary to adopt complex measures simply to pursue high electromagnetic interference suppression indicators. Generally, the strength of electromagnetic interference suppression capability is directly proportional to investment. Electromagnetic compatibility (EMC) of variable frequency speed control electrical systems is a highly complex systems engineering project, requiring the summarization of much practical experience and the exploration of much theory. With the continuous application of new technologies and theories, we believe that EMC problems will be effectively solved.
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