Harmonics are caused by devices that draw non-sinusoidal currents when a sinusoidal voltage is applied. Many times these are devices which convert AC to DC. Some of the devices which cause harmonics are listed below:
- Adjustable Speed Drives (ASDs)
- DC Drives
- Variable Frequency Drives (VFDs)
- 6-pulse Converters
- Power Rectifiers (e.g., plating systems)
- Uninterruptible Power Supplies (UPSs)
The devices listed above use power electronics such as SCRs, diodes and thyristors. These devices have become a growing percentage of the load in industrial power systems. The majority use a 6-pulse converter similar to that in Figure 3.
Loads which cause harmonics do so as a steady-state phenomenon. Therefore, if a load is suspected to be non-linear, then even an instantaneous reading of the load (while it is operating) can determine if it is harmonic producing.
Each type of load would typically exhibit a specific harmonic spectrum. For example, the most common industrial harmonic source is the 6-pulse converter. It exhibits a spectrum starting with the 5th harmonic and decreasing in amplitude throughout its spectrum. This spectrum is defined in the following formula and corresponding graph (Figure 4):
h = np ± 1, where:
- h = harmonic numbers of the spectrum
- n = 1,2,3, ...
- p = 6 for a 6-pulse converter
Therefore h = 5, 7, 11, 13, 17, 19, 23, 25, ...
Another common spectrum is that of a switch-mode power supply used for personal computers. This is found in commercial applications and has a spectrum starting with the 3rd harmonic and continuing with the triplens as the most dominant.
h = 3, 9, 15, 21, 27, ...
Large UPS (Uninterruptible Power Supply) systems exhibit yet another typical spectrum. They tend to use a 12-pulse converter and have the following spectrum:
h = np ± 1, where:
- h = harmonic numbers of the spectrum
- n = 1, 2, 3, ...
- p = 12 for a 12-pulse converter
Therefore, h = 11, 13, 23, 25, 35, 37, ...