Harmonic Distortion

Harmonic distortion has become an increasing concern for facility managers, users of automation equipment and specifying engineers alike. Harmonics not only waste energy, but they reduce equipment life, electrical system reliability, system efficiency and equipment productivity.

Harmonics can cause all kinds of trouble such as conductors overheating, nuisance tripping of breakers, sensitive equipment malfunction… Without the proper measurement or protection, you may not even be aware of what’s causing the problems.

• Harmonics are distortion currents produced by non-linear load such as the AC/DC power converters and Variable speed/Frequency Drives. Total harmonic distortion (THD) is commonly used to quantify how much of a waveform is distorted by harmonics.

• Harmonic contents can be represented in details by harmonic spectrum graphs as shown below. The harmonic contents are measured in multiples of the supply fundamental frequency. For example, on a 50 Hz supply
  - Fundament frequency = 50Hz
  - 5th harmonic = 250 Hz
  - 7th harmonic = 350 Hz.
  
  Ideal Sinusoidal waveform:
  No harmonic content. Low THD
  
  Typical input current waveform with inverter/VFD drives:
  
  Harmonic content present at 5th, 7th, 11th, 13th, 17th and 19th order. High THD
  

• Hong Kong's harmonic (THD) standards and requirements specified by the EMSD department. Please refer to the table and links:
  Code of practice for Energy Efficiency of Building Services Installation
  Code of practice for Energy Efficiency of Electrical installation
  

• There are a number of approaches to suppress harmonics, but the most cost effective method is by installing a passive harmonic filter. Passive harmonic filters are tuned to specifically reduce the current at multiple orders of harmonic frequencies and consists of only passive components, L inductor and C capacitor. Therefore well-designed passive harmonic filters are very reliable and can last for many years.

• Actual harmonic test results:
  
  VFD Harmonic Current distortion (THD = 81.1%)
  
  
  Harmonic Current Distortion with passive Harmonic Filter (THD = 6.5%)
  
  
  * VFD Frequency inverter drive: Toshiba VFPS1-4185PL-WN (18.5kW)
  Loading condition: ~80% of rated loading
  

• Typically passive harmonic filters can also improve power factor of the system to above 0.95 under nominal loading conditions. Therefore both of Hong Kong’s utility companies (CLP Power and Hong Kong Electric) power factor requirements can be fulfilled without additional components or cost.

• When power factor is improved, reactive power is reduced so overall system efficiency is improved.
  
  
• If you have any concerns or would like to verify the harmonic performance, please contact us to schedule on-site testing. We provide power quality test services and reports to help you optimize harmonic filter performance to meet your needs.