Harmonics are electrical voltages or currents that occur at frequencies that are integer multiples of the fundamental frequency of the power system. In alternating current (AC) systems, the fundamental frequency is typically 50 Hz or 60 Hz, depending on the region. Harmonics can distort the shape of the voltage or current waveform, leading to various problems in electrical systems, including increased losses, overheating of equipment, reduced power quality, and interference with sensitive electronic devices.

Common sources of harmonics include non-linear loads such as:

1. Switch-mode power supplies: Found in computers, LED lighting, variable frequency drives (VFDs), and other electronic equipment.
2. Rectifiers and inverters: Used in power electronics equipment like adjustable-speed drives, uninterruptible power supplies (UPS), and photovoltaic (PV) systems.
3. Arc furnaces: Used in industrial applications for melting metals.
4. Lighting ballasts: Found in fluorescent lamps and other discharge lighting systems.

To reduce harmonics and mitigate their negative effects, several techniques can be employed:

1. Filtering: Passive or active harmonic filters can be installed to attenuate harmonics generated by non-linear loads. These filters typically consist of inductors, capacitors, and/or active components that cancel out harmonic currents or voltages.

2. Isolation Transformers: Isolation transformers can help mitigate harmonics by isolating sensitive loads from the harmonics generated by other equipment in the system. However, this method may not be effective for high levels of harmonics.

3. Use of Line Reactors: Installing line reactors or chokes in the power supply lines can help reduce harmonics by limiting the rate of change of current and voltage, thus mitigating harmonic distortion.

4. Active Power Factor Correction: Active power factor correction (PFC) techniques, such as the use of active front-end rectifiers, can help reduce harmonics by controlling the input current waveform and improving power factor.

5. Load Management: Distributing non-linear loads across multiple phases or staggering their operation can help reduce the concentration of harmonics on individual phases and mitigate their effects.

6. Harmonic Resistant Equipment: Selecting and using equipment that is designed to produce fewer harmonics can help reduce harmonic distortion in the system. This includes choosing energy-efficient electronic devices with low harmonic content.

7. Compliance with Standards: Following relevant standards and regulations, such as IEEE 519 and IEC 61000-3-2, can help ensure that harmonic levels in electrical systems remain within acceptable limits.

By implementing these measures, harmonics can be reduced, leading to improved power quality, increased reliability, and reduced risks of equipment damage in electrical systems.