Abstract: High-order harmonics can interfere with nearby electrical and electronic equipment, causing them to overheat, shorten their lifespan, or malfunction, such as accidental operation. Therefore, analyzing and studying the harmonic interference generated by frequency converters in application and formulating corresponding anti-interference countermeasures is of great practical significance.
Due to their excellent speed regulation performance and significant energy-saving effect, frequency converters are widely used in industries such as petrochemicals, metallurgy, and machinery as speed control devices for asynchronous motors. These frequency converters typically employ an AC-DC-AC speed regulation method. However, because they utilize internal rectifier-inverter circuits, they inevitably generate high-order harmonics. These harmonics can interfere with nearby electrical and electronic equipment, causing them to overheat, shorten their lifespan, or malfunction, such as accidental operation. Therefore, analyzing and studying the harmonic interference generated by frequency converters in application and developing corresponding anti-interference countermeasures is of significant practical importance.
The mechanism of harmonic generation in frequency converters
A frequency converter has four basic components: a rectifier section, an intermediate DC link, an inverter section, and a control section. The input side of the frequency converter is a rectifier circuit, which has nonlinear characteristics and therefore inevitably generates high-order harmonics.
Generally speaking, if the rectifier circuit is a 6m-phase rectifier circuit composed of m three-phase bridge rectifier circuits, its power supply side current will contain 6m±1 harmonics (m=1, 2, 3, ...). When the frequency converter is connected to the power supply, these high-order harmonics will pollute the power supply. The inverter section on the output side of general-purpose frequency converters mostly adopts sinusoidal pulse width modulation, i.e., SPWM. Although this modulation method greatly reduces the harmonic components on the output side compared to other control methods, it still contains high-order harmonic components. These harmonics will cause overheating problems in electrical equipment, electronic equipment, intermediate DC links, inverter sections, and control sections . The root cause is that the frequency converter generates a certain harmonic, which excites the resonance of the resonant circuit formed by the capacitor and other parts.
The impact and hazards of frequency converters on the power supply system: The harmonics generated by frequency converters on the power transmission and distribution equipment, electrical equipment, measuring instruments and other facilities of the power supply system are detailed in the table and diagram. The harmonic interference generated by frequency converters and their hazards: Because frequency converters generate harmonics during operation, these harmonics can interfere with the power system's power transmission and distribution equipment, electrical equipment, measuring instruments and other facilities, causing them to malfunction or become inoperable.
The following analysis explains the harmonic interference generated by frequency converters during use and its hazards from the perspective of electricity consumption in factories and enterprises.
The impact and hazards on the power supply system of a factory: For factories and enterprises, their power supply system consists of transformers, transmission lines, substation equipment (such as switches and busbars), and electrical equipment (such as motors). Among them, the transmission lines for petrochemical enterprises are mostly power cables.
This system naturally has distributed capacitance. In addition, in order to improve the power factor of the plant power supply system, capacitors for reactive power compensation are generally installed. These capacitors will form a series or parallel resonant circuit with other parts of the system (mostly inductive circuits) at a certain resonant frequency.
The frequency converter connected to the power supply system happens to be a harmonic source. When the frequency of a certain harmonic generated by the frequency converter is equal to or close to the resonant frequency of the series or parallel resonant circuit mentioned above, the system will resonate. Resonance will bring dangerous overvoltage and overcurrent, causing equipment failure or endangering its safe operation.
For example, a factory once experienced an overcurrent in the compensation capacitor connected to the same busbar after the frequency converter was put into operation. Anti-interference countermeasures: As can be seen from the above analysis, frequency converters generate various harmonics during use. These harmonics will cause various interferences to the corresponding or adjacent electrical and electronic equipment and measuring instruments, affecting their normal operation, such as overheating, false tripping, false operation, and inaccurate measurement by measuring instruments. We believe that the following different countermeasures can be taken to reduce the harmonic interference and hazards generated by the frequency converter in different situations.
(1) First of all, when selecting a frequency converter, you should choose those frequency converters with low harmonic content, especially low-order harmonic content, on the input and output sides. For example, choose those frequency converters with multi-pulse rectifier circuits in the input rectifier section and modulation method in the output inverter section.
For the power supply system of factories and enterprises, the total capacity of frequency converters used in the factory should be fully considered to ensure that the total capacity of the frequency converters matches the total capacity of the power supply system in the factory, and that the harmonic content in the power supply system does not exceed the provisions of the Provisional Regulations on Harmonic Management of Power Systems.
(3) For high-power frequency converters, an isolation transformer should be installed on the power supply side to reduce harmonic pollution to the power supply. For electronic equipment, in order to prevent harmonic interference, it is also best to install an isolation transformer on the power supply side.
(4) Harmonic interference generated by frequency converters mainly affects and interferes with electronic equipment and radio communication facilities through radiation, electromagnetic induction, electrostatic induction, and line propagation. Different anti-interference measures can be taken to reduce the impact and interference on electronic equipment and communication facilities in response to different propagation modes of interference signals.
(5) When measuring circuits connected to frequency converters, in order to reduce measurement errors, it is best to select different instruments based on the different input and output quantities of voltage, current and power being measured.
Harmonics generated by transformer frequency converters increase the transformer's hysteresis loss, eddy current loss, and copper loss. The higher the harmonic frequency, the greater the loss. Increased losses will cause severe overheating of the transformer, increasing the burden on the transformer insulation. Harmonics generated by the frequency converter at frequencies multiples of the fundamental frequency will form circulating currents in the transformer windings, causing winding overload. At the same time, harmonics will also increase the transformer's noise.
The flow of harmonic current in power cables, including circuit breakers, power capacitors, motors, electromagnetic relays, induction relays, relay protection devices, and rectifier relays, not only causes additional losses in the cable but also creates voltage spikes, damaging the cable and accelerating the aging of the cable insulation. The higher the rated voltage of the cable, the greater the harmonic hazard. If the current flowing through the circuit breaker contains a large number of harmonics, the current at the zero-crossing point may be much larger than the normal value, which will reduce the breaking capacity of the circuit breaker and greatly increase the chance of its contacts burning out.
Harmonics cause additional losses in power capacitors and distort voltage waveforms, generating voltage spikes that damage capacitor insulation. Resonance occurs when the harmonic frequencies generated by the inverter are equal to or close to the resonant frequencies of a series or parallel resonant circuit formed by the capacitor and other system components. This can cause the capacitor to malfunction, overheat, or be damaged. Harmonics increase the iron and copper losses and additional losses in motors. Furthermore, harmonic current flowing into the motor causes additional torque, which in turn generates mechanical vibration and noise. Mechanical vibration is extremely damaging to motors and significantly impacts their lifespan.
When the current flowing through the relay coil contains harmonic current, the effective value of each harmonic current, including the fundamental wave, will be proportional to the relay's operating torque. Thus, when the harmonic current exceeds a certain value, it will affect the relay's operating value. Generally speaking, because electromagnetic relays have relatively large inertia, as long as the content of harmonic current does not exceed a certain limit, the impact is generally not significant.
Induction relays operate by driving the rotation of a disc or cylinder based on the principle of electromagnetic induction. When the current flowing through the relay coil contains harmonic current, the rotation of the disc or cylinder will vibrate, but because the inertia of the rotating parts is relatively large, it generally does not cause a significant impact.
Rectifier relays are more susceptible to harmonics than electromagnetic or induction relays. Because there are many types of rectifier relays with different operating principles, they are affected by harmonics to varying degrees. For example, incremental relays that reflect instantaneous values and high-frequency differential protection and differential protection devices that use the integral phase ratio principle are easily affected by harmonics.
The algorithms of digital relays and microprocessor-based protection devices rely on sampled data and zero-crossing, making them susceptible to harmonic interference.
The pointer deflection angle of an electromagnetic instrument is proportional to the square of the effective value of the current flowing through the coil. For an electromagnetic ammeter, if the current flowing into the meter contains harmonics, the measurement result is the effective value of the total current including the harmonic current. For an electromagnetic voltmeter, because the voltage coil has many turns and a large inductance, the high-order harmonic current flowing through the coil is not large. Therefore, even if the measured voltage contains harmonics, the measurement result is still close to the fundamental value.
Relay protection device measuring instruments, specifically rectifier-type instruments, rectify the AC quantity to be measured into DC before being measured by electromagnetic DC instruments. The deflection angle of the pointer in such electromagnetic instruments is proportional to the average value of the measured current. Since the waveform coefficient (ratio of effective value to average value) of a sinusoidal quantity is constant, the scale of this type of instrument can be proportionally calibrated to the effective value. However, when measuring non-sinusoidal quantities, because the waveform coefficients of sinusoidal and non-sinusoidal quantities are different, while the average value is accurate when using a rectifier-type instrument to measure non-sinusoidal quantities, the pointer reading may not be accurate.
Thermoelectric instruments measure current by passing it through a hot wire and measuring the temperature generated by the wire. The measurement result is proportional to the square of the measured current. Therefore, when measuring non-sinusoidal quantities, the measurement result of this type of instrument is the effective value of the measured current, which includes harmonic current.
Electric digital voltmeter electronic devices are mainly used to measure electrical power. Since the average value of the product of higher harmonic current and fundamental voltage is zero, the pointer deflection of this instrument is only proportional to the active power (the power of the fundamental current), and it is not greatly affected by harmonics.
Digital voltmeters measure voltage by sampling and analog-to-digital conversion. Both their sampling and conversion elements are designed for power frequency voltage, and they sample the peak values of a series of pulses. However, when using a digital voltmeter to measure the output voltage of a frequency converter, the converter's output voltage is a modulated series of pulses. The average voltage is adjusted by changing the duty cycle between pulses, and the modulation frequency is 1/2. This causes malfunctions in the digital voltmeter's sampling and analog-to-digital conversion, resulting in inaccurate measurements.
When the capacity of the variable frequency speed control system is large enough, the high-frequency signals generated can interfere with various surrounding electronic devices through radiation, electromagnetic induction, and electrostatic induction circuits. This can affect the normal reception of wireless equipment, the normal operation of surrounding electronic devices, causing them to receive signals incorrectly and malfunction, or affect the detection of sensor circuits and cause incorrect judgments.
