Harmonics - What’s that ????
•Harmonics are waveform distortions ( current and voltage ) caused by the irregular switching action of non linear loads
•Current waveforms from non-linear loads appear distorted because the non-linear waveform is the result of adding harmonic components to the fundamental current.
•Non-linear loads generate high levels of harmonic currents and when supplying power to these loads, a special transformer design is necessary.
•Of these non-linear loads, the major source of harmonic currents is the switch mode power supply found in most desktop computers, terminals, data processors and other office equipment.
•Harmonics are produced by the diode-capacitor input section of power supplies. The diode-capacitor section rectifies the AC input power into the DC voltage used by the internal circuits. The personal computer uses DC voltage internally to power the various circuits and boards that make up the computer. The circuit of the power supply only draws current from the AC line during the peaks of the voltage waveform, thereby charging a capacitor to the Peak of the line voltage. The DC equipment requirements are fed from this capacitor and, as a result, the current waveform becomes distorted.
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•The harmonics in the electric power distribution system combine with the fundamental (50 Hz) to create distortion. The level of distortion is directly related to the frequencies and amplitudes of the harmonic current. All of the harmonic frequency currents combine with the fundamental current to form the total harmonic distortion. (THD) The THD value is expressed as a percentage of the fundamental current and any THD values over 10% are significant enough for concern.
•Wherever there are large numbers of non-linear loads, there are harmonics in the distribution system. It is not uncommon for THD levels in industrial plants to reach 25%-30%. Normally, THD levels in office settings will be lower than in industrial plants, but office equipment is much more susceptible to variations in power quality.
•IEEE standard 519-1992 is a guidance document for utilities and electric power users which specifies both the maximum distortion levels and recommends correction levels. The harmonic distortion limit of 5%-7% is proven to be the point where harmonics begin to have a detrimental effect on the electrical distribution system.
Hazards of harmonics !!!
Typical Hazards are :
•Overheating of supply cables and conductors.
•Overheating of electrical motors and alternators, shortening life expectancy and increasing maintenance costs.
•“SKIN EFFECT” resulting in higher costs for thicker supply cables to prevent losses and cable insulation breakdown.
•Zero crossover dependant equipment such as clock-timers or watt-hour meters are negatively affected by harmonics causing crossover offset.
•False MCCB tripping can be a result of harmonic disturbance and causes catastrophic failure of mission critical equipment.
•Interference on computer networks, telecommunications, broadcasting equipment and television or video display are often the result of harmonics at audio or radio frequencies
Harmonic remedies
•Most Transformerless UPS’s have low THD characteristics whilst transformer based UPS’s can have THD values of up to 32%.
•A well designed UPS with tuned LC filtration will reduce, and in most cases, eliminate harmonic threats.
•12 Pulse UPS’s or UPS’s with power factor correction/THD filtration will reduce harmonics to acceptable levels.
•Specify and/or check your UPS suppliers specifications to ensure that THD is limited to between 5 and 10 percent maximum for critical applications.
•IF NOT… YOU MAY SOON HAVE YOUR HARMONIC GENERATING EQUIPMENT SWITCHED OFF BY AUTHORITIES!!
