Harmonics are by-products of modern electronics and automation. They are especially prevalent where there are large number of personal computers, adjustable speed drives and other types of equipment that draw currents in a non-linear manner (short pulses).
The consequences of harmonic distortion in our commercial and industrial establishments are not necessarily evil, but an issue that we must live with and manage. If we must automate, produce optimally and be efficient.
We shall look at what are harmonics, how are they generated, the effects, culprits as well as the cost implications.
What they are
Harmonics are multiples of a fundamental frequency. They cause problems when combined with the fundamental electrical waveform. When harmonics mix with the fundamental, they distort the sine wave.
How they are generated?
Devices that conduct current for less than the entire voltage sine wave are non-linear loads, and consequently generate harmonics. This includes any device with a rectifier, and pulse-generating devices such as VFD’s, Electronic ballasts, Electronic test equipment, and Switched-mode power supplies.
Since harmonic current flowing through system impedances, generates harmonic voltage distortion, creating voltage drops.
In severe instances, this voltage distortion can cause thermal tripping of relays and protective devices, and logic faults in PLCs and VFDs. As voltage distortion increases, linear loads begin to draw harmonic current.
In Motors, some of these harmonic currents-most notably the fifth and eleventh harmonics-create counter-torque in the motor, causing it to draw more current, which decreases motor efficiency, increases heating, and shortens motor life.
Measuring Harmonics
Total Harmonic Distortion (THD) is a measure that tells how much distortion of a voltage or currents is due to harmonics in a signal.
Vn-rms – is the RMS voltage of the nth harmonic
Vfund-rms – is the RMS voltage of the fundamental frequency.
Harmonic distortion is a normal consequence of a power system supplying electronic loads such as computers, business machines, electronic lightning ballasts and control systems.
Consequences of Distortion
Harmonic distortion can cause:
- High current to flow in neutral conductors.
- Motors and transformers to run hot and shorten their lifespan.
- Increased susceptibility to voltage sags, potentially causing spurious resets.
- Reduced efficiency of transformer-or, a larger unit is required to accommodate harmonics.
- Audible noise.
Cost Implications
The major costs are associated with shortened life of motors and transformers. If the equipment is part of production systems, income will be affected as well.
Example
Assume the cost to replace a 100KVA transformer is #2,000,000 including labor. Assume 100 hours of downtime each year with income loss of #50.000 per hour.
Total cost: #2,000,000 + (100 x 50,000) = #7,000,000 annually
Culprits
Motors, Transformers and Neutral conductors serving electronic loads.
How much is Ok?
Voltage distortion (THD) should be investigated if it is over 5% on any phase. Some current distortion (THD) is normal on any part of the system serving electronic loads. Acceptable limit of THD can be found in IEEE-519 (2014)
Monitor current levels and temperature at transformers to be sure that they are not overstressed. Neutral current should not exceed the capacity of the neutral conductor.
At FullSpectrum Energy, we help businesses mitigate the effects of harmonics, prolong the lifespan of their Motors, Transformers and other equipment.
Saving you headaches of overheating, nuisance tripping of relay and resets, reduced downtime and losses, thus enhancing seamless productivity and profits.
Enquiries
Victor Oyedu, FNSE, FNIEEE, CPQ.
Power Quality and Energy Management Specialist.
Publisher at Afrienergyonline.com &
CEO, FullSpectrum Energy Solutions Limited, Nigeria.
