How to Reduce Harmonics in Electrical Power Systems

Harmonics in electrical power systems are undesirable voltage and current distortions caused by nonlinear loads. These distortions can lead to overheating, increased losses, malfunction of sensitive equipment, and reduced efficiency in electrical networks. Understanding how to mitigate harmonics is crucial for maintaining power quality and ensuring the reliability of electrical systems.

This article will provide an in-depth technical explanation of harmonics, their effects, and practical methods to reduce them, including detailed calculations and examples.

Understanding Harmonics in Electrical Systems

What Are Harmonics?

Harmonics are integer multiples of the fundamental frequency (50 Hz or 60 Hz). They are produced by nonlinear loads such as variable frequency drives (VFDs), rectifiers, transformers, and power electronic devices.

For example, in a 50 Hz system:

 

• 1st Harmonic = 50 Hz (fundamental)
• 2nd Harmonic = 100 Hz
• 3rd Harmonic = 150 Hz
• 5th Harmonic = 250 Hz

Odd harmonics (3rd, 5th, 7th) are more dominant in power systems, whereas even harmonics are typically less significant due to system symmetry.

Causes of Harmonics

• Nonlinear loads (e.g., computers, LED lighting, UPS systems)
• Saturation of transformers
• Electronic power converters (rectifiers, inverters)
• Arc furnaces and welding machines

Effects of Harmonics

• Increased heating in transformers and motors
• Voltage distortion leading to malfunctioning of sensitive equipment
• Reduced efficiency due to higher losses in electrical components
• Nuisance tripping of circuit breakers and protective devices
• Interference with communication signals

Methods to Reduce Harmonics

1. Passive Harmonic Filters

Passive harmonic filters consist of inductors (L), capacitors (C), and resistors (R) designed to block specific harmonic frequencies. They are typically designed for 5th, 7th, and 11th harmonics, which are common in industrial loads.

Design Calculation of a Passive Filter

A single-tuned filter is designed as follows:

$f_{res} = \frac{1}{2\pi \sqrt{LC}}$

where:

• $f_{res}$ = Resonant frequency (Hz)
• $L$ = Inductance (H)
• $C$ = Capacitance (F)

For a 5th harmonic filter in a 50 Hz system: $f_{res} = 5 \times 50 = 250 \text{ Hz}$ Choosing an appropriate capacitor, the inductor is calculated accordingly.

2. Active Harmonic Filters (AHFs)

AHFs dynamically inject compensating currents to cancel out harmonics. These filters use power electronics and digital control algorithms to achieve near-instantaneous correction.

Example of Active Filter Application

A manufacturing plant with 100 kW nonlinear load experiences high THD (Total Harmonic Distortion) of 15%. An AHF is installed, reducing THD to 3%, significantly improving power quality.

3. Phase Shifting Transformers

Phase shifting transformers alter the phase angles of harmonic components, effectively reducing triplen harmonics (3rd, 9th, 15th). These are commonly used in industrial and commercial setups.

4. Use of Multi-Pulse Converters

Multi-pulse rectifiers (12-pulse or 18-pulse) are designed to minimize harmonic distortion by phase cancellation.

Calculation Example: For a 12-pulse rectifier, the lowest significant harmonic is calculated as: 

$h = np \pm 1$ 

where:

• $n$ = integer
• $p$ = number of pulses (e.g., 12) Thus, the lowest harmonics are 11th (550 Hz) and 13th (650 Hz) in a 50 Hz system.

5. Increasing System Impedance

Adding reactors or using transformers with higher impedance reduces harmonic currents.

Example: A 10% impedance reactor in a VFD input line can significantly lower harmonic content.

6. Load Balancing and Power Factor Correction

Maintaining balanced loads and using power factor correction capacitors can mitigate harmonics. However, improper capacitor selection may lead to resonance issues.

7. IEEE 519 Compliance

IEEE 519 provides harmonic limits for power systems. Following its guidelines ensures acceptable THD levels.

Example Calculation for Harmonic Mitigation

Problem Statement: A factory operates with a 3-phase rectifier load of 200 kW at 400 V, causing a THD of 20%. Determine the required active filter capacity to reduce THD to 5%.

Solution:

1. Apparent Power (S): $$

   $S = \frac{200}{0.9} = 222.22 kVA$ (assuming a power factor of 0.9)

2. Harmonic Current: ${ }_{}$

   $I_h=I_{fund}\times \frac{THD}{100}$

   $I_h = I_{fund} \times \frac{THD}{100}$ $I_h=321A\times \frac{20}{100}=64.2A$

   $I_h = 321 A \times \frac{20}{100} = 64.2 A$

3. Required Filter Compensation: 

   $I_{comp} = I_{h} - (I_{fund} \times \frac{5}{100})$ ${ }_{}$

   $I_{comp} = 64.2 A - (321 A \times 0.05) = 48.15 A$

Thus, an AHF rated for at least 50 A is recommended to meet the 5% THD requirement.

Frequently Asked Questions (FAQs)

Q1: What is the acceptable limit for harmonics?

IEEE 519 suggests that voltage THD should be below 5% and current THD should be below 20% for industrial systems.

Q2: Can power factor correction capacitors eliminate harmonics?

No, capacitors can help improve power factor but may resonate with harmonics if not designed properly.

Q3: Which is better: passive or active filters?

Active filters are more efficient for dynamic loads, while passive filters are cost-effective for stable harmonic sources.

Q4: How do I measure harmonics in my system?

Use a power quality analyzer or harmonic analyzer to measure THD and individual harmonic components.

Q5: Do LED lights contribute to harmonics?

Yes, LED drivers contain power electronics that can introduce harmonics into the system.

Conclusion

Harmonics pose significant challenges in electrical power systems, but with the right mitigation techniques—such as passive/active filters, phase shifting transformers, and multi-pulse converters—harmonic distortion can be minimized. Following industry standards like IEEE 519 ensures power quality and system reliability.

By implementing these methods, industries can enhance efficiency, prevent equipment failures, and reduce unnecessary energy losses.