The effects of three-phase harmonics

The effects of three-phase harmonics on circuits are similar to the effects of stress

and high blood pressure on the human body. High levels of stress or harmonic distortion can lead to problems for the utility's distribution system, plant distribution

system and any other equipment serviced by that distribution system. Effects can

range from spurious operation of equipment to a shutdown of important plant

equipment, such as machines or assembly lines.

Harmonics can lead to power system inefficiency. Some of the negative ways that

harmonics may affect plant equipment are listed below:

❏ Conductor Overheating: a function of the square rms current per unit volume of the conductor. Harmonic currents on undersized conductors or cables

can cause a "skin effect", which increases with frequency and is similar to a

centrifugal force.

❏ Capacitors: can be affected by heat rise increases due to power loss and reduced life on the capacitors. If a capacitor is tuned to one of the characteristic

harmonics such as the 5th or 7th, overvoltage and resonance can cause dielectric failure or rupture the capacitor.

❏ Fuses and Circuit Breakers: harmonics can cause false or spurious operations and trips, damaging or blowing components for no apparent reason.

❏ Transformers: have increased iron and copper losses or eddy currents due

to stray flux losses. This causes excessive overheating in the transformer

windings. Typically, the use of appropriate "K factor" rated units are recommended for non-linear loads.

❏ Generators: have similar problems to transformers. Sizing and coordination is critical to the operation of the voltage regulator and controls. Excessive harmonic voltage distortion will cause multiple zero crossings of the

current waveform. Multiple zero crossings affect the timing of the voltage

regulator, causing interference and operation instability.

❏ Utility Meters: may record measurements incorrectly, resulting in higher

billings to consumers.

❏ Drives/Power Supplies: can be affected by misoperation due to multiple

zero crossings. Harmonics can cause failure of the commutation circuits,

found in DC drives and AC drives with silicon controlled rectifiers (SCRs).

❏ Computers/Telephones: may experience interference or failures.