What is harmonics in electrical circuits?

Answer 1

By simple definition, harmonics are any "non-linear" current or voltage in an electrical distribution system. An electrical harmonic shouldn't be thought of as an acoustic or vibrational harmonic, but simply as any electrical device that draws current un-proportionally to voltage. Harmonics are commonly produced by devices that rectify AC Voltages into a DC Voltages. Common harmonic producing devices are VFDs, lighting, and computers.

When do harmonics cause problems?Current Harmonics

These important bridge rectifier circuits produce current harmonics. Current harmonics do have an affect on the electrical equipment supplying harmonic current to the device (Transformers, conductors). Current harmonics can cause issues with distribution equipment which has to handle the current from the utility transformer all the way down to the device, but generally don't affect other equipment connected to the electrical system. Harmonic currents can cause excessive heating to transformers. For electrical systems feeding single phase loads the third harmonic has gained attention in design consideration and transformer selection for causing the neutral conductor to draw excessive current.

Voltage Harmonics

Voltage harmonics can effect sensitive equipment throughout your facility. Voltage harmonics arise when current harmonics are able to create sags in the voltage supply. When any device draws current it creates a voltage dip which is required for current to flow. This voltage dip is visible with larger loads when turning on a hair dryer or a table saw and seeing the lights dim down. The amount of sag depends on many factors like transformer impedance wire size. Current harmonics create voltage harmonics, but the magnitude of the voltage harmonics depends on the "stiffness" of your electrical distribution's "system impedance."

An example to help understand current distortion verse voltage distortion is the common CFL light bulb. This low cost light bulb may have a 75% current THD (total harmonic distortion). This means that 75% of the current drawn by the bulb is considered "harmonic current". These light bulbs usually do not affect other devices in your home because even though the current drawn by the bridge circuit is rich in harmonic current, it creates very little sag in your home's voltage supply, and if you had a voltage analyzer attached you would probably see a voltage THD of well less than 1 percent.

Avoiding Harmonic Issues

If possible it is best to select equipment with a low current THD, and the result would be reduced voltage THD. If you are not able to purchase equipment with a low current THD there may be options available to you like adding line chokes or isolations transformers to reduce the harmonic currents. If you are unable to reduce current distortions and are having issues with the affects of voltage distortion there are options, such as tuned capacitor to supply harmonic current, or possibly redesigning your systems distribution to reduce system impedance.

Solutions for Mitigating Harmonics

There are many solutions to alleviating the affects of harmonics, below are some commonly applied solutions.

Reduce Current Harmonics

Add line chokes to harmonic producing equipment.

Add isolation transformer to harmonic producing equipment.

Use 12-pulse or 18-pulse rectifier circuits, instead of 6-pulse.

Reduce Voltage Harmonics

Add tuned capacitor banks to supply harmonic current, or change transformer size/impedance.

Other

Isolate sensitive loads instead of mitigating harmonics.

Answer 2

WhAt is the question? It happens in the electrical grid. It's just like it occurs in any other medium. There are filtering systems that minimize that effect.

Answer 3

due unbalance load in the system