A 12-pulse rectifier is mainly used in large UPS systems (typically ≥300 kVA) to reduce input current harmonics and improve power quality. Here’s why it is chosen for these applications:
1. Reduction of Harmonic Distortion (Compliance with IEEE 519)
· Standard 6-pulse rectifiers generate high levels of 5th and 7th harmonic distortion, leading to poor power quality.
· 12-pulse rectifiers significantly reduce these harmonics by introducing phase shifting, which cancels out certain harmonic orders, especially the 5th and 7th harmonics.
· This helps large UPS systems comply with IEEE 519 and other harmonic distortion limits, ensuring minimal impact on the power grid.
2. Better Power Factor and Reduced Transformer Stress
· 6-pulse rectifiers draw non-linear currents, causing low power factor and higher THD (Total Harmonic Distortion).
· 12-pulse rectifiers improve the input power factor and reduce harmonic currents, reducing the burden on upstream transformers and power distribution systems.
· This is critical in large-scale industrial applications where transformers and generators must handle large loads efficiently.
3. Use of Phase-Shifting Transformers
· 12-pulse rectifiers require a phase-shifting transformer, typically with Δ-Δ and Δ-Y windings, to create a 30-degree phase shift between two rectifier bridges.
· This cancels out specific harmonic orders (mainly the 5th and 7th), making the UPS system more grid-friendly and avoiding excessive harmonic filter requirements.
· Why in large UPS (>300 kVA)? Because these transformers are bulky and expensive, making them impractical for smaller UPS systems.
4. Improved Generator Compatibility
· Large UPS systems (≥300 kVA) often operate with diesel generators in data centers, hospitals, and industrial plants.
· High harmonic currents from 6-pulse rectifiers can cause voltage distortion and instability in generator operation.
· A 12-pulse rectifier reduces harmonic currents, allowing stable generator operation with minimal derating.
5. Legacy vs. Modern UPS Systems
· Before IGBT rectifiers, the 12-pulse rectifier was the primary method for reducing harmonics in large UPS systems.
· Modern UPS systems use IGBT-based rectifiers (Active Front End - AFE), which achieve low THD (<3%) without the need for 12-pulse transformers.
· However, some legacy large UPS systems (300 kVA - 1 MVA+) still use 12-pulse rectifiers, especially in industrial settings where harmonic compliance is crucial.
Conclusion
A 12-pulse rectifier is used in large UPS systems (≥300 kVA) because it:
✅ Reduces harmonic distortion (especially 5th and 7th harmonics)
✅ Improves power quality, ensuring compliance with IEEE 519
✅ Enhances power factor and minimizes impact on upstream transformers
✅ Improves generator compatibility, reducing the risk of voltage distortion
✅ Was historically necessary before IGBT-based rectifiers became common
🔹 For new designs, many manufacturers now prefer IGBT-based rectifiers over 12-pulse due to better efficiency and lower cost.
