Harmonics emission

Electronic equipment and devices that cause nonlinear current waveforms produce many detrimental effects on the ac power distribution. As more of these devices load the ac power distribution system, the following problems can occur:

• Significant current can flow that does not deliver real power and must be supported by the ac distribution system;
• Elevated neutral currents can flow in three-phase wire distribution systems which cause ampacity ratings for wiring and connectors to be exceeded;
• Equipment connected to the same branch circuit can operate improperly due to severe voltage distortion caused by harmonic currents interacting with impedance's in the distribution system. For example, suppressed or clipped peak voltage limits the ability of computer power supplies to ride through the momentary ac mains sags.

Equipment manufacturers generally use power factor correction (PFC) circuitry to minimize the effect of nonlinear loads and the power inefficiencies they cause. Testing to the harmonic current regulations helps to ensure these PFC circuits are designed and operating properly.

The harmonic current distortion caused by nonlinear load on the ac mains can be classified in one of two general categories: quasi-stationary or fluctuating.

• Quasi-stationary or steady-state harmonic currents are produced by electronic equipment that generates non-varying levels of current distortion, where the amplitude of each harmonic remains constant over time. This equipment appears as an unchanging load on the ac mains;
• In contrast, fluctuating harmonic currents are caused by electronic equipment that represent time-varying loading on the ac mains. This equipment generates varying levels of current consumption or drain where the amplitude of individual harmonics change over time. Examples include microwave ovens, dishwashers, laser printers and photocopiers.