A byproduct of modern electronics, harmonic distortion occurs when large numbers of personal computers, uninterruptible power supplies, variable-frequency drives (VFDs), and other devices using solid-state power-switching supplies to convert alternating-current (AC) power to direct-current (DC) power are present. In HVAC systems, AC drives are the most prevalent source of harmonic distortion.

Harmonic distortion can be present in both voltage and current. In a balanced three-phase power system, even-numbered harmonic waveforms are 120 degrees out of phase and cancel each other, while odd-numbered harmonic waveforms remain. For a three-phase rectifier load, odd-numbered harmonic currents are expressed in the following equation: 6 × n ± 1, such as 5, 7, 11, 13, etc. The magnitude of the sinusoidal waveform for harmonic currents decreases as frequency increases and vice versa.

Harmonic distortion does not add power to a system, even though additional current flows through electrical wires. The effects of three-phase harmonic distortion on circuits are similar to the effects of stress and high blood pressure on the human body. High levels of harmonic distortion can lead to problems for distribution systems and the equipment they serve. The effects can range from spurious operation to power-system inefficiency to equipment shutdown.

The negative effects of harmonic distortion on equipment include:

• Conductor overheating. Harmonic currents on undersized conductors or cables can cause a “skin effect” that increases with frequency and is similar to a centrifugal force.

• Blown fuses and circuit breakers. Harmonic distortion can cause false or spurious operation and trips, damaging or blowing components.

• Reduced capacitor life. Heat-rise increases attributed to power loss can reduce the life of capacitors. If a capacitor is tuned to one of the characteristic harmonics, overvoltage and resonance can cause dielectric failure or rupture a capacitor.

• Transformer overheating. Increased iron and copper losses, or eddy currents, attributed to stray flux losses cause excessive overheating in transformer windings.

• Unstable generator operation. Excessive harmonic voltage distortion causes multiple zero crossings of current waveforms. Zero crossings affect the timing of voltage regulators, causing interference and operational instability.

• Incorrect utility-meter readings. Incorrectly recorded measurements can result in higher utility bills.

EVALUATING SYSTEM HARMONICS

System harmonics should be evaluated in the event of one or more of the following:

• Capacitor banks are applied in an HVAC system in which 20 percent or more of the load includes other harmonic-generating equipment.

• The facility has a history of harmonic-related problems, including excessive capacitor-fuse operation.

• Power-company requirements severely limit the harmonics back into a facility's system.

• Plant expansions have resulted in significant harmonic-generating equipment operating in conjunction with capacitor banks.

• Plans call for the addition of an emergency standby generator as an alternate power source in an industrial facility.

Often, the supplier of non-linear-load equipment, such as VFDs, can evaluate the effects equipment can have on an HVAC system. Usually, this involves details related to HVAC-system design and impedance.