Cont'd WHAT ARE HARMONICS?

Methods of Harmonic Mitigation

Methods of harmonic mitigation include:-

• Phase Shifting;
• Passive Filters;
• Phase Staggering;
• Active Filters.

Phase Shifting

• Most common is a phase shift transformer to create two supplies displaced in phase by 30º from the original supply. By feeding two 6-pulse diode rectifiers to create a 12-pulse rectifier, certain harmonics will be opposite in phase and magnitude, and will cancel each other out.
• Solutions up to quasi 48-pulse can be created.

Passive Filters

• Passive filters are series capacitor & reactor resonant circuits ‘tuned’ to present a low impedance path to a specific frequency (i.e. 5th - 250Hz, 7th - 350Hz).
• They are more commonly used at the PCC to prevent harmonics ‘escaping’ into the utility power supply, but can be connected to individual loads in the plant.
• Due to possible system resonance and interaction problems, a harmonic study of the power system is often necessary. If multiple filters are installed anti-resonance is a problem. Install with caution !
• Typical total harmonic attenuation to ~ 10-15%