SOURCES

If these harmonic currents flow in a power system, then they cause what is known as poor "power quality" or "dirty power". There are other causes of poor power quality, including transients such as voltage spikes, surges, sags, and ringing. As shown in Figure 1, harmonics are considered a steady-state cause of poor power quality because they repeat every cycle. Figure 2 shows the resultant wave form from a 5th harmonic present in a 60 Hz system.

Figure 1 - Fundamental and 5th Harmonic	Figure 2 - Fundamental and 5th Harmonic Combined

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, ...

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