Until recently, almost all circuit loads were linear, with the current wave-form closely matching a sinusoidal voltage wave-form and changing in proportion to the load. Lately, non-linear loads that use current in pulses and at a frequency other than 60-Hz have increased dramatically.
Examples of linear and non-linear devices are:
Linear devices
Non-linear devices
The increase in non-linear loads has led to harmonic distortion in electrical distribution systems. Although capacitors do not cause harmonics, they can aggravate existing conditions. The existence of harmonic currents is a site-specific problem. It results from the complex interrelationship between all the electrical/electronic equipment in a facility and, therefore, is difficult to predict and model.
A brief explanation of Variable Frequency Drives (VFD) helps explain the problems associated with harmonics. A VFD uses switching power supplies to control output. In a six-step VFD, the control switches six times per cycle in an effort to simulate a sine wave. As the time between switches changes, the motor receives varying apparent frequencies and changes synchronous speed accordingly.
These changes in apparent frequencies lead to two problems: large voltage spikes and distorted current wave shapes. The voltage spikes are short and generally won't affect equipment that doesn't use a zero crossing of the voltage wave shape for timing. The distorted current wave shape is the "harmonic generator"
Harmonics cause additional noise on the line, and this noise generates heat. The increase in heat can cause circuit breaker nuisance trips. Power factor capacitors experience the same problem. Their fuses tend to blow due to thermal overload. In fact, these are some of the early warning signs that harmonic currents exist in the facility.
