Halting harmful harmonics: Page 3 of 4

April 10, 2018 // By John Mitchell
Halting harmful harmonics
The rise of non-linear loads in industrial environments over the last two decades has resulted in the growing problem of harmonic currents and utility-level voltage distortion. Facing a lack of awareness, the industry has struggled to implement effective mitigation techniques. 

Shunt-passive

Shunt passive is power factor correction, often using fixed capacitor banks, tuned and detuned contactor based units, thyristor capacitor banks and fine-tuned passive filters. These methods were principally developed to resolve reactive power and not specifically for harmonic mitigation. Today, I would hope no one is installing capacitor banks by themselves and, at the very least, using de-tuned ones – with an inductor for example.

An inherent weakness of passive solutions is the inability to control the load. The grid loading, along with the filter's impedance, can cause several fine-tuned shunt filters to interact, resulting in resonance with other equipment.

 

Series-active

Series-active takes the form of an Active-Front-End (AFE) variable speed drive (VSD). It replaces the rectifier diodes in a regular VSD with an IGBT controlled rectifier to eliminate switching based signal noise. This circuitry also allows the AFE to introduce regenerative braking. Although this unit may at first seem to eliminate harmonics, it must be noted that with the AFE in addition to the VSD, there are now two drives in the circuit producing heat.

This means twice the heat and with a 200kW AFE it soon adds up. For the panel builder / system integrator, bigger cooling systems are needed to cope with the excessive heat. In one application where AFE was offered against a VSD and active filtering, the payback in heat loss alone was 2.5 years.

AFEs are great at significantly lowering THD and maintaining good power factor. However AFEs have some serious drawbacks. In order to maintain a small form factor, lower switching frequencies are used, which result in high switch ripples on the voltage waveform. This can cause equipment to nuisance trip and malfunction. AFEs are generally a plant level solution; separate regenerative units can be used only where necessary to lower capital expenditure and increase return on investment (ROI).

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