In a perfect world, the currents flowing in an Alternating Current (AC) system are always sinusoidal, following a smooth and predictable pattern. However, real-world conditions often deviate from this ideal, leading to non-sinusoidal currents. This deviation is primarily due to the presence of harmonics. But what exactly are harmonics, how are they generated, and what impact do they have on electrical systems and equipment?

What are Harmonics?

Harmonics in an electrical system refers to voltage or current components at frequencies that are multiples of the fundamental frequency. For most AC systems, the fundamental frequency is either 50 Hz or 60 Hz, depending on the region.

• 1st harmonic: The fundamental frequency (e.g., 60 Hz).
• 2nd harmonic: Twice the fundamental frequency (e.g., 120 Hz).
• 3rd harmonic: Three times the fundamental frequency (e.g., 180 Hz).

Harmonics distort the ideal sinusoidal waveform, resulting in a non-sinusoidal waveform.

The graph above illustrates the fundamental sine wave and the 3rd harmonic. The distortion caused by the 3rd harmonic results in the overall waveform deviating from the ideal sine wave.

How Are Harmonics Generated?

Harmonics are primarily generated by non-linear loads in the electrical system. Non-linear loads draw current in a non-linear manner, meaning the current waveform does not follow the voltage waveform. Common sources of non-linear loads include:

• Power Electronics: Devices such as rectifiers, inverters, and variable frequency drives (VFDs).
• Electronic Equipment: Computers, printers, and other office equipment.
• Lighting: Fluorescent lamps and LED lights with electronic ballasts.
• Uninterruptible Power Supplies (UPS): Systems providing emergency power to loads.

These devices operate by creating sharp changes in current, which generate harmonics.

Effects of Harmonics on Systems and Equipment

Harmonics can significantly impact electrical systems and equipment, leading to several detrimental effects:

1. Overheating: Harmonics increase the RMS (Root Mean Square) value of the current, causing additional heating in electrical equipment such as transformers, motors, and cables. This overheating can reduce their lifespan and lead to premature failure.
2. Reduced Efficiency: The presence of harmonics can cause losses in electrical machines and equipment, reducing their efficiency and increasing operating costs.
3. Interference: Harmonics can interfere with communication lines and sensitive electronic equipment, leading to malfunctions or erroneous operations.
4. Overloading: Distribution systems can become overloaded due to the increased RMS current, potentially causing protective devices like circuit breakers to trip unnecessarily.
5. Resonance: Harmonics can lead to resonance conditions, where the system’s impedance becomes very high at certain frequencies, exacerbating the effects of harmonics and leading to voltage distortion and equipment damage.

Focus on 3rd Harmonics

The 3rd harmonic, which is three times the fundamental frequency, is particularly problematic in three-phase power systems. In a 60 Hz system, the 3rd harmonic frequency would be 180 Hz. 3rd harmonics are of zero sequence, meaning the 3rd harmonic components in each phase are in phase with each other.

Generation of 3rd Harmonics:

3rd harmonics are often generated by:

• Single-phase non-linear loads: Multiple single-phase loads connected to a three-phase system can cause the 3rd harmonic component from each phase to add up in the neutral conductor.
• Electronic ballasts in lighting: These often draw non-sinusoidal current that includes significant 3rd harmonic content.

Effects of 3rd Harmonics:

1. Neutral Overloading: In a three-phase system, the neutral current can become excessively high because the 3rd harmonic currents from each phase add up rather than cancel out. This can lead to overheating of the neutral conductor and even cause fires.
2. Transformer Heating: Transformers can experience increased heating due to 3rd harmonics. In particular, delta-wye transformers can have circulating currents in the delta winding due to 3rd harmonics, leading to additional losses and heating.
3. Voltage Distortion: 3rd harmonics can cause significant voltage distortion, leading to malfunction of sensitive equipment and misoperation of protective relays.
4. Interference: Harmonics can cause interference with communication lines and other electronic equipment, potentially disrupting operations and causing data errors.

Mitigating Harmonics

To maintain the reliability and efficiency of electrical systems, mitigating harmonics is crucial. Here are some common methods:

• Passive Filters: Used to block or filter out specific harmonic frequencies.
• Active Harmonic Filters: Devices that actively inject currents to cancel out harmonic components.
• K-Rated Transformers: Designed to handle the additional heating caused by harmonics.
• Proper Load Management: Distributing non-linear loads more evenly across phases and minimizing their usage can reduce harmonic generation.

Understanding and managing harmonics is essential to ensuring the longevity of equipment, maintaining system reliability, and preventing potential safety hazards. By addressing harmonics, we can move closer to achieving the ideal sinusoidal current flow in AC systems.