Common Sources of Harmonics in Electrical Networks

The main source of the harmonics is any non-linear loads that produce the voltage harmonics and current harmonics. The load is said to be nonlinear when the current it draws does not have the same waveform as the supply voltage. Mostly, this load is represented by the device comprising power electronics circuits such as:

Transformers
Rotating machines
Power converters
Variable frequency drives
Fluorescent lamps
Arcing devices
Uninterruptible power supplies (UPS)
Switched-mode power supplies
Office equipment and household appliances

Sources of Harmonics

The sources of harmonics are:

1. Transformers

A transformer can incur core saturation conditions in either of the following cases: When operating above-rated power when operating above-rated voltage the first situation can arise during peak demand periods, and the second case can occur during light load conditions, especially if utility capacitor banks are not disconnected accordingly and the feeder voltage rises above nominal values.

A transformer operating on the saturation region will show a nonlinear magnetizing current, which contains a variety of odd harmonics, with the third dominant. The effect will become more evident with increasing loading. No hysteresis losses are produced in an ideal lossless core. The magnetic flux and the current needed to produce them are related to the magnetizing current of the steel sheet material used in the core construction. Even under this condition, if we plot the magnetizing current vs. time for every flux value considered, the resultant current waveform would be far from sinusoidal. Sinusoidal magnetizing current is not symmetrical with respect to its maximum value because the hysteresis effect is considered. The distortion is typically due to triplen harmonics (odd multiples of three, namely, the 3rd, 9th, 15th, etc.), but mainly due to the third harmonic. This spectral component can be confined within the transformer using delta transformer connections. This will help maintain a supply voltage with a reasonable sinusoidal waveform.

2. Rotating machines

Due to rotor slots or slight irregularities in the winding patterns of a three-phase winding of a rotating machine or small asymmetries on the machine, harmonic currents can develop. These harmonics induce an electromotive force (emf) on the stator windings at a frequency equal to the ratio of speed/wavelength. The resultant distribution of magnetomotive forces (MMF) in the machine produces harmonics, which is the function of speed. Additional harmonic currents can be created upon magnetic core saturation. However, these harmonic currents are usually smaller than those developed when the machines are fed through variable frequency drives (VFDs).

3. Power converters

The increasing use of the parameters like voltage and frequency are varied to adapt to specific industrial and commercial processes has made power converters the most widespread source of harmonics in distribution systems. Electronic switching helps the task to rectify 50/60 Hz AC into DC power. In DC applications, the voltage is varied by adjusting the firing angle of the electronic switching device. Basically, in the rectifying process, the current is allowed to pass through semiconductor devices during only a fraction of the fundamental frequency cycle, for which power converters are often regarded as energy-saving devices. If energy is to be used as AC but at a different frequency, the DC output from the converter is passed through an electronic switching inverter that brings the DC power back to AC.

4. Variable frequency drives

VFDs are, in reality, power converters. The reason to further address them under a separate section is that, by themselves, VFDs constitute a broad area of application used in diverse and multiple industrial processes.

In a very general context, two types of VFDs can be distinguished: those that rectify AC power and convert it back into AC power at a variable frequency and those that rectify AC power and directly feed it to DC motors in several industrial applications. In both cases, the front-end rectifier, which can make use of diodes, thyristors, IGBTs, or any other semiconductor switch, carry out the commutation process in which current is transferred from one phase to the other. This demand of current “in slices” produces significant current distortion and voltage notching right on the source side, i.e., at the point of common coupling. Motor speed variations, which are achieved through firing angle control, will provide different levels of harmonic content on the current and voltage waveforms. Variable frequency drive designs also determine where harmonic currents will predominantly have an impact. For example, voltage source inverters produce complex waveforms showing significant harmonic distortion on the voltage and less on the current waveforms. On the other hand, current source inverters produce current waveforms with considerable harmonic contents with voltage waveforms closer to sinusoidal. None of the drive systems is expected to show large distortion on both voltage and current waveforms.

5. Fluorescent lamps

Electronic lighting ballasts have become popular in recent years following claims for improved efficiency. Overall, they are only a little more efficient than the best magnetic ballasts and in fact, most of the gain is attributable to the lamp being more efficient when driven at high frequency rather than to the electronic ballast itself. Their chief advantage is that the light level can be maintained over an extended lifetime by feedback control of the running current – a practice that reduces the overall lifetime efficiency. Their great disadvantage is that they generate harmonics in the supply current. So-called power-factor corrected types are available at higher ratings that reduce the harmonic problems but at a cost penalty. Smaller units are usually uncorrected.

Compact fluorescent lamps (CFL) are now being sold as replacements for tungsten filament bulbs. A miniature electronic ballast, housed in the connector casing, controls a folded 8mm diameter fluorescent tube. CFLs rated at 11 watts are sold as replacements for a 60-watt filament lamp and have a life expectancy of 8000 hours.

These lamps are being widely used to replace filament bulbs in domestic properties and especially in hotels where serious harmonic problems are suddenly becoming common.

6. Arcing devices

The voltage-current characteristics of electric arcs are highly non-linear. Following arc ignition, the voltage decreases due to the short-circuit current, the value of which is only limited by the power system impedance. The main harmonic sources in this category are the electric arc furnace. It discharges type lighting with magnetic ballasts.

The harmonics produced by electric arc furnaces are not predicted due to variations in the arc feed material. The arc current is highly nonlinear and reveals a continuous spectrum of harmonic frequencies of both integer and non-integer order. Due to the physical Phenomenon of melting with a moving electrode and molten material, the arc furnace load gives the worst distortion. The arc current wave may not be the same from cycle to cycle.

7. Uninterruptible power supplies (UPS)

Uninterruptible power supply systems are usually used to provide “secure power” in the event of generator shutdown or other similar power failures. Dedicated individual computer UPS systems are usually single-phase and have an input current wave-shape and harmonic current spectrum similar to that produced by single-phase switched-mode power supplies (SMPS). Three-phase UPS systems are also available. The majority of three-phase UPS systems have a controlled, SCR input bridge rectifier with characteristic harmonics based on the “pulse number ± 1” format.

8. Switched-mode power supplies

These power supplies are the “front end” of single-phase 120V loads such as PCs and home entertainment equipment. Typically, they have a full-wave diode rectifier connected between the AC supply system and a capacitor, and the capacitor serves as a low-ripple “battery” for the DC load. Unfortunately, low ripple means that the AC system charges the capacitor for only a fraction of each half-cycle, yielding an AC waveform that is highly peaked.

9. Office equipment and household appliances

Many types of non-linear loads appeared and their usage rate increased rapidly. Non-linear loads TVs, video recorders, computers, printers, microwave ovens, discharge lighting, vapor mercury, halogen spotlight, and halogen with dimmers became widely used these days besides the rapid increase of industrial non-linear loads.

The sources of harmonics in the electrical system of the future will be various and numerous. Due to the increasing use of sensitive electronics in personal computers, multimedia, digital communications, and industrial automated processes, the problem grows complicated.