Methods for reducing Harmonics in System

---

Methods for Reducing Harmonics in Electrical Systems

Harmonics are one of the biggest challenges in modern electrical systems, especially with the growing use of non-linear loads such as Variable Frequency Drives (VFDs), rectifiers, and electronic devices. Excessive harmonics can lead to equipment overheating, reduced efficiency, nuisance tripping, and overall poor power quality.

Fortunately, several methods exist to reduce harmonics and improve system reliability. Let’s explore the most effective solutions.

---

1. DC Choke

• Application: Commonly used in VFDs (a major source of harmonics).
• Working Principle: A DC choke is an inductor in series with the DC link of the semiconductor bridge circuit.
• Effect:
• Reduces 5th and 7th order harmonics
• Improves current waveform smoothness
• Comparable to AC-side line reactors (though THD reduction is slightly less).

✅ Key Point: DC chokes are simple, cost-effective, and widely used in drives.

---

2. Reducing Harmonics at Loads and Source

Since VFDs and non-linear equipment are major harmonic sources, minimizing harmonics directly at the load/source is often the most effective strategy.

Example – Transformer Phase Shifting

• Installing a Delta-Star Transformer in parallel with a Delta-Delta Transformer allows conversion of two synchronized 6-pulse VFDs into a 12-pulse VFD application.
• Why effective?
• Phase shift changes from 60° (6-pulse) to 30° (12-pulse).
• Reduces 5th and 7th harmonics.
• Delta connection blocks zero-sequence harmonics → eliminating triple harmonics.

✅ Key Point: Transformer configuration plays a big role in harmonic mitigation.

---

3. Using Filters

Filters are one of the most common methods to control harmonics. They are broadly divided into passive and active filters.

(a) Passive Filters

• Made of inductors, capacitors, and transformers.
• Work by blocking or diverting harmonics to ground.
• Designed for specific harmonic orders.

Advantages:

• Simple and cost-effective.
• Widely used in AC drives (line reactors, transformers).

Limitations:

• Effectiveness reduces when harmonic spectrum changes with load variation.
• Can cause resonance issues if not designed properly.

---

(b) Active Filters

• Also called power line conditioners.
• Work by sensing harmonic currents and injecting a counter waveform to cancel them out.
• Installed in parallel with the load.

How They Work:

1. Current Transducers measure the load current.
2. Fundamental frequency component is removed.
3. The remaining harmonic waveform is inverted and injected back via IGBTs (PWM switching).
4. Harmonics are cancelled → improving both power factor and voltage waveform.

Key Components:

• Power semiconductors (IGBTs, PWM control)
• DC link capacitors & bus bars
• Internal filters & protective fuses

Scalability: If harmonic levels exceed one filter’s rating, multiple active filters can be installed in parallel.

✅ Key Point: Active filters are more flexible and effective than passive filters, especially for varying load conditions.

---

Quick Comparison

Method	Best For	Harmonic Orders Reduced	Cost & Complexity
DC Choke	VFDs	5th, 7th	Low
Transformer Shift	Industrial plants (multi-VFDs)	5th, 7th, triple	Medium
Passive Filter	Fixed load harmonics	Specific orders	Low–Medium
Active Filter	Variable loads, precise mitigation	Wide range (dynamic)	Higher

---

Conclusion

Harmonics cannot be eliminated entirely, but they can be controlled and minimized with the right mix of DC chokes, transformer configurations, and filters. For most industrial setups, a combination approach works best:

• DC chokes for VFDs
• Transformer phase-shifting for multiple drives
• Filters (passive or active) for overall system compliance with IEEE/IEC harmonic limits.

By applying these solutions, power quality improves, equipment life extends, and energy efficiency increases.

---