Reducing Harmonics in AC Drive Systems through Structural Modifications and Filtering Methods

A Variable Frequency Drive (VFD), also known as an Variable Speed Drive (VSD), is an electronic device that controls the speed and torque of an electric motor by varying the frequency and voltage of the power supplied to it. While VFDs offer many benefits, they can also introduce harmonic distortion in the electrical system, which can lead to several issues.

Harmonic distortion occurs when the frequency of the power supplied by the VFD does not match the fundamental frequency of the power grid. This can cause harmonic currents to flow back into the power grid, which can cause voltage fluctuations, overloading of electrical equipment, and even damage to sensitive electronic devices.

The sources of harmonic distortion in VFDs include:

Power supply distortion - The power supplied to the VFD can contain harmonics, which can be amplified by the VFD.
Non-linear loads - Non-linear loads, such as rectifiers and inverters, can generate harmonics that can be amplified by the VFD.
Voltage reflection - When the VFD switches on and off, it can cause voltage reflections in the power system, which can lead to harmonic distortion.

The effects of harmonic distortion caused by VFDs include:

Overheating - Harmonic currents can cause the overheating of electrical equipment, such as transformers and motors.
Equipment failure - Harmonic distortion can cause premature failure of electrical equipment, reducing its lifespan.
Voltage instability - Harmonic distortion can cause voltage instability, which can lead to flickering lights and malfunctions in other electrical devices.

To mitigate the effects of harmonic distortion, it is important to use appropriate mitigation techniques, such as using harmonic filters, selecting suitable VFDs, and following good design practices.

How VFD's Generate Harmonics

Variable Frequency Drives (VFDs) generate harmonic distortion because they are electronic devices that use power electronics to convert AC power to DC power, and then back to AC power at a different frequency and voltage. This process involves a series of switching operations that create waveforms that contain harmonic frequencies in addition to the fundamental frequency. The harmonic frequencies are multiples of the fundamental frequency.

The primary sources of harmonic distortion in Variable Frequency Drives (VFDs) are rectifiers, inverters, and Pulse Width Modulation (PWM). VFDs use a rectifier to convert AC power to DC power, and diodes to chop off the negative half cycles of the input waveform. This results in a pulsating DC voltage waveform that contains harmonics that are multiples of the fundamental frequency.

Figure 1. Order of Harmonic Current | Source: ABB

Inverters are used to convert the DC voltage back to AC voltage at a different frequency and voltage level. This process involves switching the DC voltage on and off rapidly using a series of transistors or thyristors, creating a stepped waveform that contains high-frequency harmonics beyond the 25th harmonic.

Additionally, VFDs use PWM to control the output voltage and frequency, which involves the use of high-frequency pulses to control the amplitude and timing of the output voltage. This technique results in the generation of high-frequency harmonics that can also extend beyond the 25th harmonic. The harmonic distortion generated by VFDs can cause problems in the electrical system, such as overloading of electrical equipment, overheating, equipment failure, voltage instability, and interference with electronic equipment. Therefore, appropriate mitigation techniques should be used to address the effects of harmonic distortion.

Reducing VFD Harmonics by Structural Modifications

Harmonics reduction can be done either by structural modifications in the drive system or by using external filtering. The structural modifications can be to strengthen the supply, to use 12 or more pulse drive, to use a controlled rectifier or to improve the internal filtering in the drive.

Figure 2. Drive System Features Affecting Harmonics | Source: ABB

Figure 2 shows the factors in the AC drive system which have some influence on harmonics. The current harmonics depend on the drive construction and the voltage harmonics are the current harmonics multiplied by the supply impedances.

Using 6-pulse diode rectifier

A 6-pulse diode rectifier is a commonly used rectifier configuration in AC drive systems. It uses a single-phase transformer and six diodes to convert AC power to DC power. However, it generates significant harmonic distortion, specifically the 5th and 7th harmonics. To reduce these harmonics, a technique called "harmonic filtering" can be used. This involves the addition of capacitors and inductors in the circuit to filter out the unwanted harmonics.

Figure 3. 6-pulse diode rectifier | Source: ABB

Using 12-pulse or 24-pulse diode rectifier

A 12-pulse or 24-pulse diode rectifier is a more complex rectifier configuration that uses two or four 6-pulse diode rectifiers in parallel. This configuration reduces the amount of harmonic distortion significantly compared to a 6-pulse diode rectifier. Specifically, a 12-pulse diode rectifier reduces the 5th and 7th harmonics, while a 24-pulse diode rectifier reduces the 5th, 7th, 11th, and 13th harmonics. This reduction in harmonic distortion reduces the amount of electrical noise and improves the power factor of the system.

Using phase-controlled thyristor rectifier

A phase-controlled thyristor rectifier uses thyristors instead of diodes to convert AC power to DC power. The thyristors can be controlled to change the phase angle of the AC input voltage. By changing the phase angle, the harmonic distortion can be reduced. Specifically, by adjusting the firing angle of the thyristors, the 5th and 7th harmonics can be reduced. However, phase-controlled thyristor rectifiers are less efficient than diode rectifiers and may require additional cooling.

Using IGBT bridge

An IGBT (Insulated Gate Bipolar Transistor) bridge is an electronic device used in AC drive systems to convert AC power to DC power. IGBTs are more efficient than diodes and thyristors and can be controlled to reduce harmonic distortion. The IGBT bridge can be used with a DC link capacitor and an AC input filter to further reduce harmonic distortion. The AC input filter, consisting of inductors and capacitors, helps to filter out high-frequency harmonics that are generated by the IGBT bridge. The DC link capacitor helps to smooth out the DC voltage and reduce ripple current, further reducing harmonic distortion.

Figure 4. Comparison | Source: ABB

Using larger DC or AC inductor

The use of a larger DC or AC inductor in an AC drive system can help to reduce harmonic distortion. Inductors store energy in a magnetic field and can be used to smooth out the DC current in the circuit. A larger inductor can provide a smoother DC current and reduce ripple current, reducing harmonic distortion. Similarly, an AC input inductor can be used to smooth out the AC current, reducing the harmonics generated by the rectifier. The use of larger inductors can increase the cost and size of the AC drive system, but it can also improve the power quality and efficiency of the system.

Figure 5. The Effect of Inductor in Line Current | Source: ABB

Reducing VFD Harmonics by External Filtering

Another effective method to reduce harmonics in AC drive systems is to use filtering techniques. A harmonic filter is an electrical device that is designed to attenuate specific harmonics in the AC power system, thereby reducing harmonic distortion. There are several types of harmonic filters that can be used, depending on the specific application and the harmonic frequencies that need to be attenuated.

One common type of harmonic filter is a passive filter, which uses inductors, capacitors, and resistors to attenuate harmonics. Passive filters can be designed to target specific harmonic frequencies and can be used in series or parallel with the VFD to reduce harmonic distortion. Another type of harmonic filter is an active filter, which uses power electronics to actively monitor the harmonic content of the load and generate a counteracting waveform that cancels out the harmonics. Active filters can be more effective than passive filters at reducing harmonic distortion, particularly at high-frequency harmonics.

Figure 6. External Active Filter | Source: ABB

When choosing a harmonic filter or multi-pulse VFD, it is important to consider the specific harmonic frequencies that need to be attenuated, as well as the cost and practicality of the system. A harmonic analysis of the electrical system can help identify the specific harmonic frequencies and guide the selection of the appropriate harmonic filter or multi-pulse VFD.

Reducing harmonics in variable frequency drives (VFDs) is essential for maintaining compliance with regulatory standards, improving power quality, extending equipment lifespan, and achieving energy savings. Harmonics generated by VFDs can cause issues with power quality, including voltage fluctuations and overheating of transformers, as well as interference with other equipment. Non-compliance with regulatory standards can result in penalties or fines, and in some cases, the VFD may need to be replaced with a unit that meets the regulations.

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Title: Guide to Harmonics with AC Drives
Source: ABB

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