How can Harmonics in a VFD affect Power Quality?

What is VFD?

VFD – Variable Frequency Drives

VFDs, which stand for variable frequency drives, are utilized extensively in industrial settings for the purpose of controlling the speed of the electric motors. 

In comparison to conventional methods of motor control, variable frequency drives (VFDs) provide a number of advantages, including 

• Increased energy economy, 
• Enhanced process control, and 
• Decreased wear and strain on machinery. 

What is Harmonics?

Harmonics can be produced by variable frequency drives (VFDs), which can lead to issues for the electrical system and the equipment that is connected to it. 

This post will discuss the technical factors that lead to harmonics, the difficulties that they might cause, and possible solutions that are available for reducing the effects of harmonics caused by variable frequency drives (VFDs).

Harmonics are currents or voltages that occur at frequencies which are multiples of fundamental frequency of power system, which is typically between 50 and 60 hertz while it is in operation. 

Non-linear loads, that draw current in a manner that is not sinusoidal, are the thing that causes harmonics to occur. 

Electric variable frequency drives (VFDs) are a typical source of non-linear loads in the industrial applications. 

In the process of switching, the variable frequency drive (VFD) generates a waveform known as pulse-width modulated (PWM), which is characterized by the presence of high-frequency harmonics. 

When the switching frequency of the variable frequency drive (VFD) is increased, the order of harmonics that are created also increases.

Harmonics Problems

It is possible for harmonics to cause a number of issues for power system as well as the equipment that is linked to it. These are some of the frequently often encountered issues:

1. Overheat
2. Failure of VFD component due to Overheating
3. Voltage Distortion
4. Resonance
5. Power Factor Reduction
6. Interference with Communications Systems
7. Malfunction
8. Excessive Neutral Current 
9. Torque Pulsations

Overheat

There is a possibility that harmonic currents could cause transformers, motors, & other parts of equipment to overheat, which will result in a shorter lifespan and higher maintenance expenses.

Failure of VFD component due to Overheating

Components of variable frequency drives (VFD) such as capacitors and transistors can overheat and fail prematurely.

Voltage Distortion

The waveform of the voltage can be distorted by harmonic currents, which can result in faults in sensitive equipment such as computer & control systems which are sensitive to them. 

Resonance

Harmonics have the potential to introduce resonance into the power system, which can result in voltage surges and malfunctions of equipment.

Power Factor Reduction

There is a possibility that harmonics will lower the power factor of the system, which will result in higher energy expenses and fines.

Interference with Communications Systems

Interference with many communication systems, including 

• Wireless Fidelity (Wi-Fi), 
• Cellular radio, & 
• Radio frequency devices

Malfunction

There is a malfunction in sensitive electronic equipment such as 

• Adjustable Speed Drives, 
• Programmable Logic Controllers, and 
• Other similar devices.

Excessive Neutral Current 

An excessive amount of neutral current that triggers voltage drops, causes the transformer to overheat, and causes circuit breakers to trip for no apparent reason.

Torque Pulsations

Turbine pulsations in motors are a source of decreased efficiency and increased mechanical wear.

Harmonics Penalty: In addition to this, several governments are now committing to the imposition of harmonics penalties, which will be imposed in case the levels of harmonics are not within the acceptable range as outlined in IEEE 519 standards.

VFD Harmonic Mitigation Techniques

When it comes to minimizing harmonics caused by variable frequency drives (VFDs), there are numerous options available:

1. Line Reactors
2. Passive Harmonic Filters
3. Active Harmonic Filters
4. Multi-Pulse VFD
5. Active Front-End VFD
6. Other Mitigation

Line Reactors

It is possible to filter away harmonics by installing line reactors between the variable frequency drive (VFD) and the power supply. Devices known as line reactors are the passive devices that make use of inductance to smooth out the waveform of the current.

Passive Harmonic Filters

When it comes to use with variable frequency drives (VFDs), the most popular form of harmonic filter is a passive one. Harmonics are filtered out by installing them between the variable frequency drive (VFD) and the power supply. 

In order to smooth out the waveform of the current, passive harmonic filters are the passive devices that make use of factors such as inductance and capacitance. 

They have a high level of reliability in lowering low-order harmonics, although their effectiveness may decrease when applied to higher frequencies.

Active Harmonic Filters

Electronic devices known as active harmonic filters are created by employing active power electronics in order to filter out harmonic resonances. 

Although they are slightly more costly than passive filters, they provide superior performance at frequencies that are higher as well. 

For the purpose of filtering out particular harmonics and adjusting to variations in load and output frequency, active harmonic filters can be manually set.

Multi-Pulse VFD

Multi-pulse variable frequency drives (VFDs) make use of many pairs of power semiconductors in order to lower the order of produced harmonics. 

A 12-pulse variable frequency drive (VFD) is an example of a device that utilizes 2 sets of 6-pulse rectifiers to eliminate the fifth- and seventh-order harmonics.

Active Front-End VFD

Other Mitigation

1. You should choose with a motor that has a high power factor and a decreased susceptibility to harmonic harmonics.
2. Make certain that the variable frequency drive (VFD) is correctly grounded and insulated.
3. Keep the wiring for the VFD as short as possible to reduce interference.
4. Load the variable frequency drive (VFD) to the appropriate level in order to achieve the lowest possible harmonics.

Best Practices for Harmonic Mitigation

Depending on the particular needs of the application, the optimal approach for reducing harmonics caused by variable frequency drives (VFDs) is determined. 

Active harmonic filters are more appropriate for higher-order harmonics & applications with changing loads, whilst passive harmonic filters are often considered to be the most affordable solution for low-order harmonics. 

In certain applications, line reactors & multi-pulse variable frequency drives (VFDs) can also be effective.

Summary

Harmonics from VFDs can damage power systems and equipment. Fortunately, 

• Passive and active harmonic filters, 
• Line reactors, and 
• Multi-pulse vfds 

can reduce harmonics. 

Application needs determine the best solution.