Task 1: Identify typical sources of harmonics in a power system

Outcome P2.1

Describe the follow harmonic effects;-

a) Give a brief explanation, using appropriate diagrams, as to what is meant by the term “harmonics” in power systems.

b) Explain the harmonic problems that can arise due to the magnetising current in a transformer.

c) Identify and give a brief description of three other typical sources of harmonics in a power system.

a) Give a brief explanation, using appropriate diagrams, as to what is meant by the term “harmonics” in power systems.

Harmonics can be thought of “if explained in very simple terms”, as the pollution of the parent generated frequency. The parent frequency is the fundamental frequency, and the pollution the totalling harmonics.

A graphical illustration to show a General overview of the causes of Harmonics in power systems

Harmonics are defined (by the IEEE) as voltages or currents at frequencies that are a multiple of the fundamental frequency. In most systems, the fundamental frequency is 50

Hz. Therefore, harmonic order is 100 Hz, 150 Hz, and 200 Hz and so on. (For other countries with 60 Hz systems, the harmonic order is 120 Hz, 180 Hz, 240 Hz, etc.)

We usually specify these orders by their harmonic number or multiple of the fundamental frequency. For example, a harmonic with a frequency of 150 Hz is known as the third harmonic (50x3 = 150). In this case, for every cycle of the fundamental waveform, there are three complete cycles of the harmonic waveforms. The even multiples of the fundamental frequency are known as even-order harmonics while the odd multiples are known as the odd-order harmonics.

Harmonic currents are generated whenever a non-linear load is connected to the mains supply. The problems caused by harmonic currents include overheating of cables, especially the neutral cconductor, overheating and vibration in induction motors and increased losses in transformers.Where power factor capacitors are fitted, harmonic currents can damage them and care must be taken to avoid resonance with the supply inductance.

An important fact is that the vast majority of harmonic currents found in a distribution system are odd-order harmonics (3rd, 5th, 7th, etc.). More often than not, the sources of the harmonic currents in a distribution system are the loads in operation within that facility or building. Interestingly, these are frequently the types of loads that are the most sensitive to distortion in the current and/or voltage.

The distortion of a voltage or current can be traced to the harmonics it contains. This distortion can be produced by magnetic saturation in the core of transformers or by the switching of thyristors or IGBTs in electronics drive.

All periodic signals of frequency “f" can be represented in the form of a composite sum;

Of a sinusoidal term at frequency “f": the FUNDAMENTAL (H1). Of sinusoidal terms of which frequencies are integer multiples of fundamental H1:

the HARMONICS (Hn). Of a possible continuous component (DC component)

y (t) = h1 (t) + h3 (t)

Graphical illustration to show the addition of harmonics

Graphical illustration of a complex harmonic waveform

The spectrum of a harmonic signal. Illustrating amplitudes of the harmonics as a function of their frequency.

A graphic to show Resolution of Non-Sinusoidal Current Waveform into Harmonic Components.

It can be shown that any voltage or current waveform, either sinusoidal or non-sinusoidal, canbe resolved into its harmonic components (also known as “orders”) with varying magnitude and phase. This is graphically demonstrated in the diagram above. At any point in time in the graph, it can easily be shown that Isum = I1 + I3 + I5, or that the magnitude of the non-sinusoidal current waveform is the sum of its harmonic components.

An illustration to show the “none” superimposed wave forms

Voltage distortion (i.e., deviation from the pure sinusoidal wave-shape), is generated indirectly as a result of harmonic currents flowing through a distribution system. Excessive harmonic current can produce distorted voltage drops in the power source impedance.

A measure of harmonic content has been defined as Total Harmonic Distortion, or THD. THD is calculated by taking the square root of the sum of the squares of all harmonics above the fundamental and dividing by the fundamental amplitude. This value is then multiplied by 100% to produce a percentage. For instance, if the fundamental amplitude is 240 volts, the 3rd harmonic amplitude is 20 volts, and the 5th harmonic amplitude is 15 volts, the voltage THD can be calculated as follows:

THD voltage (%) = √ (20²+15²) X 100% = 10.42%240

PROBLEMS DUE TO HARMONICS

An observed characteristic of harmonic currents in a typical building distribution system is that odd-ordered harmonics (i.e., the 3rd, 5th, 7th, etc.) are usually larger in amplitude than even-ordered harmonics. Another observed effect is that single-phase non-linear loads, such as power electronic devices, tend to produce harmonic currents that are predominantly triple-N or multiples of 3 (i.e., 3rd, 6th, 9th, etc.). These triple-N harmonics, especially the 3rd, tend to amplify in the shared neutral conductor of 3-phase 4-wire systems. Typical problems caused by high current and voltage harmonics include;

Resonant interaction with capacitor banks that can lead to overheating and premature failure.

Overloading of conductors and transformers. Overheating of distribution system equipment (bus bars, lugs, and conductor

insulation). Harmonic fluxes in motors that lead to decreased efficiency, heating, and

excessive vibration. Nuisance tripping of molded-case electronic circuit breakers that sense peak

current. Zero-crossing synchronization problems with devices such as digital clocks.

A flow diagram to show Harmonic effects

b) Explain the harmonic problems that can arise due to the magnetising current in a transformer.

Losses in transformers are due to stray magnetic losses in the core, and eddy current and resistive losses in the windings. Of these, eddy current losses are of most concern when harmonics are present, because they increase approximately with the square of the frequency. Before the excess losses can be determined, the harmonic spectrum of the load current must be known.

By definition third harmonic currents are present in the magnetizing current (a small portion of the transformer full load current). If the transformer saturates (due to over-voltage), the harmonic distortion level of the current increases substantially.

The effects of harmonics inside the transformers involve mainly three aspects:

a) increase of iron losses (or no-load losses)b) increase of copper lossesc) presence of harmonics circulating in the windings

a) The iron losses are due to the hysteresis phenomenon and to the losses caused by eddy currents; the losses due to hysteresis are proportional to the frequency, whereas the losses due to eddy currents depend on the square of the frequency.

b) The copper losses correspond to the power dissipated by Joule effect in the transformer windings. As the frequency rises (starting from 350 Hz) the current tends to thicken on the surface of the conductors (skin effect); under these circumstances, the conductors offer a smaller cross section to the current flow, since the losses by Joule

effect increase. These two first aspects affect the overheating which sometimes causes a de-rating of the transformer

c) The third aspect is related to the effects of the triple-N harmonics (having uniform polarity harmonics) on the transformer windings. In case of delta windings, the harmonics flow through the windings and do not spread towards the network source since they are all in phase. In the case of delta, the windings correspond to a barrier for triple-N harmonics. Whatever the type of transformer it is necessary to pay attention to this type of harmonic mechanism for a correct sizing of the transformer. The triple-N harmonics are trapped and circulate in the delta primary of the transformer. Since most loads produce high levels of the 3rd harmonic (one of the triples), the harmonic content is reflected back and thus the source is reduced. The circulating harmonics in the primary of the transformer creates heat because of their higher frequencies. For this reason, a transformer that can handle the excess heat is required. This type of transformer is called a K-rated transformer

In trying to explain what is happening with the currents inside the transformer I will make reference to the two diagrams which follow. In the first, the excitation current Ie is split into two components: the magnetizing current (I μ) and the current which is proportional to the iron core power losses (Ife). These currents are displaced from each other by an angle Π/2. This displacement can be explained by means of excitation current waveform. If the coil is supplied with sinusoidal voltage the flux Φ must be sinusoidal too. Since the magnetizing characteristic B-H is nonlinear, and has a hysteresis loop, the current waveform obtained from magnetizing curve is not sinusoidal.

Illustration to show the magnetic saturation, (the right half show the production of the top have of the Hysteresis Loop

Transformer magnetisation (including hysteresis): (a) magnetisation curve;(b) flux and magnetisation current waveforms

The Hysteresis Loop

It is usual to plot the magnetization M (vertical axis) of the sample as a function of the magnetic field strength H (horizontal axis), since H is a measure of the externally applied field which drives the magnetization.

c). Identify and give a brief description of three other typical sources of harmonics in a power system.

Illustration to show a comparison of different types harmonic sources

Switched Mode Power Supplies:

Typically found in single-phase electronic devices such as computers and other business and consumer electronics, these devices use a switching regulator to precisely control the DC voltage. The input of these power supplies normally consists of a full-wave bridge rectifier and a DC filter capacitor which produces an alternating pulse current waveform rich in third harmonic. Though they are not used in large power applications, the cumulative effects of many devices may create concerns, particularly for 415/230 Volt Y (wye) systems.

Switch mode power supply rectifier

Harmonic spectrum of a typical PC

Fluorescent lighting:

These devices produce a mainly third order harmonic current on the order of 20% to 30% of the fundamental current. Electronic ballasts have slightly different characteristics but display similar levels of harmonics.

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 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. The harmonic current spectrum is shown below.

Harmonic spectrum of a Typical Compact Fluorescent Lamps (CFL)

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.

Pulse-Width Modulated Converters:

These devices use an external controller for switching the input transistors allowing the current waveform to be shaped more desirably. However, these converters are limited in power and typically used in applications less than a few hundred kilowatts.

The Wave shapes of current absorbed by some other non-linear loads.

Light dimmer or heating regulator

Three-phase rectifier with front end capacitor

Three-phase rectifier with DC filtering reactor

References

Power System Harmonics……………………………………………N. R. Watson; Jos Arrillaga

Power systems harmonics: fundamentals, analysis, and filter design…….George J. Wakileh.Harmonics and power systems………………………………………..Francisco C. De La Rosa

Power Quality and Utilisation Guide -Causes and EffectsDavid Chapmanhttp://www.copperinfo.co.uk/power-quality/power-quality-guide.shtml

Neutral Sizing in Harmonic Rich Installations Prof Jan Desmet, Hogeschool West-Vlaanderen & Prof Angelo Baggini, Università di Bergamohttp://www.copperinfo.co.uk/power-quality/harmonics/neutral-sizing.shtml

Selection and Rating of TransformersProf Jan Desmet, Hogeschool West-Vlaanderenhttp://www.copperinfo.co.uk/power-quality/harmonics/transformer-selection-rating.shtml

Pub144 - Harmonics,Transformers and K-FactorsCopper Development Association http://www.copperinfo.co.uk/power-quality/downloads/pub-144-harmonics-transformers-k-factors.pdf

Pub145 - Harmonics in PracticeCopper Development Associationhttp://www.copperinfo.co.uk/power-quality/downloads/pub-145-harmonics-in-practice.pdf

Significant issue of harmonics in power quality and how they affect electronic equipment pt 1.Hawaiian Electric Companyhttp://www.heco.com/vcmcontent/StaticFiles/pdf/Harmonics_Primer_Part_1_PROTECTED.pdf

Significant issue of harmonics in power quality and how they affect electronic equipment pt 1.Hawaiian Electric Companyhttp://www.heco.com/vcmcontent/StaticFiles/pdf/Harmonics_Primer_Part_2_PROTECTED.pdf

Understanding Power System HarmonicsProf. Mack Grady. Dept. of Electrical & Computer EngineeringUniversity of Texas at Austinhttp://users.ece.utexas.edu/~grady/Harmonics_Notes_Grady_June_2006_print.pdf

Harmonics in Power SystemsJ.L. Hernandez, Department of Electricity. IES La Laboral de La Laguna. Avda. Lora y Tamayo, 2. La Laguna. Tenerife.MA. Castro, J. Carpio and A. Colmenar. Department of Electrical, Electronic and Control EngineeringE.T.S.I.I.- U.N.E.D. C. Juan del Rosal, 12. Ciudad Universitaria. Madrid 28040. Spain.

http://www.icrepq.com/ICREPQ'09/P1.pdf