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.
Reducing the magnitude of peak current values and managing current waveforms are important to mitigating harmonic distortion. Methods of reducing harmonic current include the use of AC line reactors, DC bus chokes, 12-pulse and 18-pulse AC drives, and devices that generate AC sine waves. These methods have cost and benefit trade-offs. Specifying engineers often request a harmonic-mitigation solution that unnecessarily increases construction and installation costs. The best solution is a trade-off between acceptable harmonic distortion and acceptable cost.
Recent innovations have made AC drives with reduced harmonics and improved performance possible. Advances in the materials used in long-life plastic-film capacitors for the diode-bridge-rectifier power-conversion sections of AC drives are reducing the amount of capacitance across DC buses. New AC drives have 3 to 5 percent of the capacitance of previous-generation drives.
New variable-torque AC drives have powerful motor-control microprocessors that are programmed with motor-control algorithms designed to produce sinusoidal waveforms for centrifugal-fan and pump applications. New technology also has decreased the size of variable-speed drives and reduced dependence on external line filters, such as AC line reactors and DC chokes, for the reduction of harmonics.
Figure 1 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive without an AC line reactor. The input voltage is the orange waveform, while the input current is the purple waveform. Large spikes in the current waveform are attributed to capacitors charging and discharging. The magnitude of the current waveform peaks at about 300 amps, as the capacitors charge. The large, double-humped current waveform significantly contributes to harmonic distortion. The total harmonic-distortion current (THDI) in this example is 80 percent.
Figure 2 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive with a 3-percent AC line reactor. The input voltage is the orange waveform, while the input current is the purple waveform. The peak current reaches 190 amps and is lower because of the use of the 3-percent AC line reactor. The double-humped waveform contributes to harmonic distortion, but is reduced in comparison with Figure 1. The THDI in this example is 38 percent.
Figure 3 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive with new reduced-harmonics technology. The input voltage is the orange waveform, while the input current is the purple waveform. The peak current is 190 amps; however, the current waveform is square-shaped and more closely resembles a sinusoidal waveform because of the reduced capacitance value. The new square-shaped current waveform produces less harmonic distortion. The THDI is 33 percent. An AC drive with new reduced-harmonics technology performs as well as an AC drive with a 3-percent line reactor.
An AC drive with reduced-harmonics technology is more efficient in handling AC power and does not require an external AC line reactor or DC bus choke.
Removing the AC line reactor or DC bus choke from a drive:
Reduces the size and weight of the drive enclosure.
Reduces the cost of the drive.
Improves ease of installation.
Reduces mechanical/electrical-room wall-space requirements.
An AC drive with new reduced-harmonics technology is a cost-effective AC-drive solution.
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Harvey Eure is a product manager for enclosed variable-frequency drives for Schneider Electric. He can be reached at [email protected].