PF Harmonics Effect+on+PQ +Day2+First+Half for+Printing

8/8/2019 PF Harmonics Effect+on+PQ +Day2+First+Half for+Printing

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Tushar Mogre·s Power & Energy Consultancy. Page: 12 Day seminar Power Factor ± Electrical Supply Harmonics ± Power Quality effect due to these.




Harmonics ± Basic Characteristics regards to supply.

Analysis of Harmonics with Symmetrical Componentsmethod.

Harmonics Effects on Electrical Supply network.

Harmonics Reduction and Filtering.

Harmonic measurement techniques.







Practical AC supply systems are not the ideal systems. They have source impedances as well asthe Electrical Current carrying cables too have some impedances. Thus, output voltage too is afunction of current carried through such supply source and cables.(433Vac output transformers show 4 to 7% and Generators show 12 to 21% output impedances)

Due to such Non-Linear currents, disturbances on the voltages are bound to occur and can be a

complex function of current non-linearity.

Ideal ACVoltageSource.

P ractical (non-ideal) AC source.e.g. Transformer supply, Generator supply,UP S / Inverter Output supply

Non-Linear +

Linear Loading.

ILV1

V1=240V

IL=100A

V2=232V

V1=240V

IL=100A

V2=232V

V2

Non-Linear Load Linear Load

Harmonic currentHarmonic Voltages




Such non-sinusoidal repetitive current waveforms aregenerated by non-linear loads. Non-linear load is a circuitelement, whose current is not proportional to voltage.

Recently, loads with non-linear components have increaseddramatically. (Since year-1990 onwards). Gone up almost 20

times now than that about 20 years ago.Following are typical examples of Non-Linear Current Loads:

Variable Frequency, Voltage speed control drives (VFD) for ind. Motors.Thyristorised DC motor drives for variable speed applications.Uninterrupted Power Supplies.(UPS).Electronic Power Converters/Inverters.Medium and High frequency Induction Furnaces.

Arc Furnaces.Electronic ballasts for fluorescent lamps.Switched Mode Power Supplies used in PLCs, Household equipments etc.









In Electrical system, Harmonics can be analysed based on their

sequence analysis:zero sequence, +ve sequence and -ve sequence components:

Normally for non-linear but balanced loading, which generates current harmonics; thecurrent harmonic contents in every phase are same (identical in terms of amplitudeand phase angle).With this as basic assumption, we can see that some odd current harmonics represent

+ve sequence, some show ±ve sequence and some are zero sequence components. As harmonic frequencies are different for different harmonics, it is almost impossible toanalyse them by using phasors. In fact it can be convenient to analyse them bywaveforms .

Harmonic analysis with Symmetrical Components:

V

Ir

Iy

Ib

Zero sequencephasors createdby 3 rd, 9 th, 15 th

21 st - - - so on.Known as triplenharmonics.

Observe this 3 rd

harmonic waveform

Current harmonicswith same

AmplitudePhase angle with

regards to V.




Observe this 5 th

harmonic waveform -ve sequencephasors createdby 5 th, 11 th, 17 th

23 rd - - - so on.Observe reversePhase sequence.

Observe this 7th

harmonic waveform +ve sequencephasors createdby 7 th, 13 th, 19 th

25 th - - - so on.Observe reversePhase sequence.

V

Ir

Iy

Ib

Ir

Iy

Ib

V

Harmonic analysis with Symmetrical Components:








Harmonic effects on Electrical Supply:Core Losses in Electromagnetic Equipments:

As we have seen from the Fourier equation that higher harmonics are higher frequency components.

Lot of machines and equipments used with electrical supply system haselectro-magnetic components. The electromagnetic equipments normally have³Core Loss´ (Iron loss) component. This is mainly due to ³Eddy Current losses´and ³Hysterisis losses´.

As Eddy current losses are Proportional to square of the frequency andhysterisis losses are proportional to frequency, Normal empirical formulasuggests that with such equipments, the core loss factor is normallyproportional to frequency to the power of 1.6

i.e. ³ Core Losses w frequency 1.6 ´.

This means that harmonics on electrical supply system if they enter theelectromagnetic equipments, they increase the Core Losses in them. In shortreducing the overall efficiency of the electromagnetic equipments.




Transmission Losses in Cables:Skin effect is the tendency of electrical current to flow on the surface of theconducting material.

Skin effect is predominant in the conductors is due to AC currents. More thefrequency of AC current flowing through the conductor, more is the skin effectphenomenon is observed.

This means that effective cross-section area of current carrying conductors isless for harmonics. Higher the harmonics, (higher frequency), lower is theeffective cross-section area.

Thus, for harmonic currents, the resistivity offered by current carryingconductors is higher. It therefore increases the transmission losses in theelectrical supply system.

Harmonic effects on Electrical Supply:




Overall P F in electrical billing shows poor effects:

Normally for electrical consumer, the PF is calculated by the Electricity supply company as: _______________________ ________________________

PF = kWh / kWh 2 + kVArh 2. (where the term kWh 2 + kVArh 2 = kVAh)(Note: There may be some versions of kVArh calculations dependent on different electricity supplycompanies)

If we look at IEC or IEEE standard definition of kW power or kVAr power, in presence of harmonics,they look as:

N NkW = 7 Vn . In . Cos(Øn) and kVAr = 7 Vn . In . Sin(Øn)

n=1 n=1

Where,Vn = Voltage of ³n´ th harmonic term. (RMS value)In = Current of ³n´ th harmonic term. (RMS value)Øn = Phase angle difference between Vx and Ix sine waves.

So in presence of harmonics, the term ³kVA´ gets affected.Needless to say, the PF in presence of harmonics would be affected and will reflect it in electricitysupply metering company bills.

Additionally, the PF improvement capacitors normally placed near the source can give fundamentalwaveform VAR compensation. Thus cannot improve on poor PF due to harmonic Presence.

Harmonic effects on Electrical Supply:




What happens if the harmonics are presentin the supply system.

Harmonics present on supply system can beconsidered like a cancer, which goes ondeteriorating the electrical equipments andultimately making the equipments to fail muchbefore their life expectancy.

Without your knowing it, power (kW) consumed on such electrical installations iscausing more money burden.

Even the Spurious operations of the protection relays / equipments as well as theenergy recording equipments can take place.

Harmonics being higher frequency components, they create more electromagneticnoise which can interfere with communication equipments in near vicinity and cancause these equipments to mal-function.

Spurious blowing of fuses and tripping of circuit breakers without an apparent causecan be due to presence of harmonics.




Higher harmonic distortion levels can causefollowing problematic Issues.

Distortion of Voltages on the supply system.

Harmonics entering Electromagnetic instruments causes higher losses due to increasein core losses component. Therefore reducing the overall efficiency of the electricalsystem in an industrial environment.

Higher dielectric losses with insulating material ± stressing out the insulation andpossibly causing it to fail.

Some harmonics create a reverse rotating magnetic fields if enters into rotatingmagnetic field machines or instruments. This can cause the machine efficiencies toreduce and some instruments like disc type energy meters can record low readings.

Electricity Utility company billing is normally based on PF calculated in presence of harmonics. Harmonic power can hamper this PF and normal PF improvement capacitorscannot improve this PF.

Skin effect enhancement due to higher frequency components in harmonics. Thiscauses additional Transmission & Distribution losses.

Higher Electromagnetic noise. The higher frequency components have higher rate of change of flux. Naturally, these have higher ability for electromagnetic induction withmagnetic circuits which is normally a noise.




Higher harmonic distortion levels can causefollowing problems with some equipments.

Capacitors: The capacitors are subjected to additional power loss, overheating andreduction in life.

Cables: Neutral overloading and higher transmission losses in the cables. Thisnecessitates the over-sizing of the cables. Insulation due to higher dielectric losses getsstressed out causing the early failure of the cables.

Transformers: The transformers are subjected to additional iron & copper losses, over-

stressing of insulation, mechanical vibrations and prohibitive noise level.Motors: The induction motor life is reduced due to heat rise in the form of core losses

and operating efficiency is reduced due to ±ve sequence harmonics.

Communication Equipments: Harmonics interfere with the communication signalscausing prohibitive noise level and at times loss of information. The interference withPower Line Carrier System may cause mis-operation while accomplishing remote

switching load control and metering.Fuses & Breakers: Due to harmonics present, there may be false / spurious operation of

the Fuses / Breakers.

Utility Meters: Utility meters may either over record or under record in the presence of harmonics, causing erroneous results.




HarmonicLoad




Harmonic Reduction & Filtering:

As the Bad effects of Harmonics that are seen are like CANCER on supply and it¶s important that this cancer

is treated earlier, the better it is for the electrical equipments.

Thus,We have to use the methods for eliminating or at least minimising the presence of harmonics on supplysystems.How do we achieve this?

Usage of Electrical equipments which are harmonic free.

Find out the predominant harmonics and filter them out with a specific harmonic tunedfilter/s.

Usage of generalised harmonic filter to filter out all the type of harmonics present onsupply by usage of active harmonic filter.

All the three methods can be applied independently or can even be applied together to improve theperformance.

Let¶s take a look at each method for their merits or demerits.




Harmonic Reduction & Filtering:Usage of Equipments with reduced / no harmonics.

New type of energy saving equipments normally uses the power Semi-conductor devices. These equipments are highly energy efficient (consumeslesser kW), but draws harmonic prone current.

Now a days, there are new type of Semi-Conductor based equipments whichuses PWM (Pulse Width Modulation) method to draw the sinusoidal power.Unfortunately such equipments are higher on costs, but gives extremely goodresults in terms of reduction of harmonics.

Typical equipments that are now a days available with such facility are:Variable speed A.C. Drives.Uninterrupted Power Supplies.Switched Mode Power Supplies.Electronic ballasts for fluorescent lamps.

But the existing equipments cannot be modified to be converted to reduce theharmonics. Thus, only option of replacement sometimes makes this approach,difficult to adapt.




Harmonic Reduction & Filtering:Usage of Equipments with reduced / no harmonics.

In industrial environment, usage of Variable speed drives (AC & DC), Inductionfurnaces (Medium & High frequency) is quite common.

Most of these equipments uses 3±Phase full wave rectifiers (thyristorised or diode based).Such equipments normally are known as six pulse converters which generatesa large amount of 5 th and 7 th Harmonics.To reduce the problems of these harmonics, the scheme is used based on12 pulse converter.

5 th & 7th HarmonicCurrent.

5 th & 7th HarmonicCurrent.

11 th & 13 th HarmonicCurrent with reduced

% THD.Load

(Drives or

InductionFurnace)

By this method, the phase shifting of 30Û between star & delta winding gives theoverall effect of eliminating 5 th and 7 th harmonics and gives reduced THD valuesof 11 th and 13 th harmonics that are simpler to filter out.




Harmonic Reduction & Filtering:Selective Harmonic filtering by usage of tuned filter.

M

M

M

M Load

Load

11kV bus

Transformer

415V distribution network

5 th 7 th Harmonic sources

Consider a typical Electrical supply installation.




5 th & 7th Harmonic sources

Now Lets say a series tuned filter L-C seriesis put across a load. This filter is say tuned

at resonance frequency of 5 th harmonic. And next the 7 th

harmonic resonancefrequency filter L-C.

As L-C series filters offer zero impedance at resonanceFrequency, all harmonic currents will flow thro¶ them.

Selective Harmonic filtering by usage of tuned filter.




M

M

M

M Load

Load

11kV bus

Transformer


Important Consideration under harmonic presence:Harmonic filters should be electrically as close to source of harmonic.

P F improvement system should be as close to source of supply as possible.Capacitors used for P F improvement should be properly detuned for harmonic frequencies.





M

M

M

M Load

Load

11kV bus

Transformer


Nodal Analysis.

At various nodal points weneed to get the variousreadings of individualharmonic Voltages andCurrents.

Once these values are

known, the in between pathimpedance offered for aspecific harmonic can bedetermined.

Based on this impedancecalculations, the completegraph of network impedances

are drawn.

This is important for calculation of tuned filter offered parallel resonancewhich under any conditions tobe avoided.





Series L-C filter is the simplest form of harmonic filter.

Impedance offered by inductor and capacitor is a function of frequency.i.e. it is dependent on the harmonic no.

Inductive impedance X L = 2 T f L.Where, L ± is inductor value in Henry. (this is constant of inductor)

f ± is the frequency.

Capacitive impedance Xc = 1 / (2 T f C)Where, C ± is capacitor value in Farad. (this is constant of capacitor)

At resonance frequency of this L-C series combination,XL = Xc

@2

Tf L = 1 / (2

Tf C) ___

@ f = 1 / 2 T L.C

By such simple formulae, the harmonic filters can be calculated but consideration shouldbe for parallel resonance phenomenon.





50

150 250 350 450 550 HzX

XC

XL

Harmonic Reduction & Filtering :




50

150 250 350 450 550 HzX

Harmonic Reduction & Filtering :

The L and C values combination is dependent on:

Fixed kVAr value to be offered by it for fundamental.

How effectively it should filter with regards to mains frequency variation.

Last but not the least ± RESONANCE condition avoidance.

XC

XL





Resonance avoidance:

Consider this supply network.There are various impedances in the network.

By basic circuit laws, we can get the equivalent circuit.Voltage sources short circuit and current source open circuit.Then calculation of equivalent impedances what we get. =We neglect the Resistance component to calculate the resonance frequency (for higher Q-factor values)We can get the exact parallel resonance frequency. The L-C filter values should be adjusted so that suchresonance frequencies near to harmonic frequencies should be avoided totally.

AC VoltageSource

Cable

Cable

CableCableCable

CableLoad 1 Load 3

Load 2

HarmonicLoad

capacitor

HarmonicFilter




In three phase supply system:

For triplen harmonic filter, the neutral currentsneeds to be compensated.In view of the same, the filters with neutralconnection are preferred.

For non-triple filters, the configuration can be deltaconnected as shown here.

Some other important considerations:

The reactors used for filters should preferably be air core reactors. These giveslinearity though-out the current variation range and does not change Henry (L) values.

The capacitors used should be capable of withstanding 2 times the normal supplyvoltage levels on continuous basis and should be stable type. APP capacitorspreferred. MPP self healing tend to change their values over time period thereforethese are not considered.






Tuned Harmonic Filter ± (Advantages & Disadvantages):Advantages:

Low Cost.Offers capacitive kVAr compensation for betterment of D- P F.Very few components ± Thus, easy for maintenance.

Rugged design.

Disadvantages:

Needs detailed Network analysis. (Nodal Analysis).Addition or subtraction of loads and modification in electrical networkcan sometimes cause the resonance problems.Normal reduction in harmonics, with properly designed system isbetween 65% to 85%. This means cannot bring down the harmonics tozero level (or near zero level).




Harmonic Reduction & Filtering:Active Harmonic Filtering.

Active Harmonic Filters uses the direct approach:

Sense the Harmonic prone current waveform.

Analyse the Fundamental part of this waveform.

Find out the waveform that is a differencebetween the actual waveform and it¶s fundamentalpart.

Make that part of the current to flow through theactive harmonic filter to give effective vector summation as a purely sinusoidal waveform.

HarmonicLoad

ActiveHarmonic

Filter





Block Diagram:

Active Harmonic Filter

HarmonicLoad

DCRectifier.

P WMInverter

There are various configurations of Active Harmonic filters based on themerits each type offers.These configurations are mainly dependent on the filtering requirementslike, balanced or individual phase filtering, device ratings in terms of currentand voltage handling etc.





Active Harmonic Filter ± (Advantages & Disadvantages):Advantages:

No resonance problems.Reduction / Elimination of wide range of harmonics with singleequipment.

Reduction of harmonics by about 85% to 97%.No need for extensive Nodal harmonic analysis and calculation of system network impedances.

Disadvantages:

Comparatively higher cost of the equipment.Higher on state working losses. i.e. higher ³kW / Harmonic current´ ratio.Complex Electronic design. Needs expert for any maintenance issues.




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t




Harmonics Measurement Technique:

We are re-writing here the Fourier Equation:f( t) 2 T =a 0 + e 1sin( t+Ø 1 ) + e 2 sin(2 t+Ø 2 ) + e 3sin(3 t+Ø 3 ) +«..+ e N sin (N t + Ø N ).

0

N N

f( t) = a 0 + 7 a n Cos (n t) + 7 b n Sin (n t) .n=1 n=1

This shows that every non-sinusoidal repetitive waveform is made up of sine and cosine wavesof fundamental frequency and it¶s higher integral multiple frequency sine and cosine waves,that are added in various amplitudes.




Average Value ± finite.

1.00

1.00 Let¶s multiply this sine wave with a unitamplitude sine wave of the same frequency.





Average Value ± zero.

1.00

1.00


Let¶s multiply this sine wave with a unitamplitude sine wave of double thefrequency.




1.00

1.00


Average Value ± zero.

Let¶s multiply this sine wave with a unitamplitude sine wave of triple thefrequency.

Similarly when Cosine wave is multipliedwith a cosine wave of unit amplitude andsame frequency, we get finite value. Butwith any other integer multiple frequency,the value is zero.




Here we make use of mathematics.If a sine wave term ³bn . sin (n t)´ is multiplied by same frequency sine wavewith unity amplitude. i.e. this term is multiplied by ³1 X sin (n t)´, then whatyou get is a DC scalar value which is finite and with average value ½ of theaverage sine wave value of ³bn . sin (n t)´.

Same way If a cosine wave term ³an . cos (n t)´ is multiplied by samefrequency cosine wave with unity amplitude. i.e. this term is multiplied by ³1 Xcos (n t)´, then what you get is a DC scalar value which is finite and withaverage value ½ of the average cosine wave value of ³an . cos (n t)´.

But if the terms ³bn . sin (n t)´ or ³an . cos (n t)´ are multiplied by unityamplitude sine or cosine waves respectively, but with frequency determininginteger other than ³n´, then the multiplication result gives average value ³zero´

± 0. Even if DC constant like ³a0´ is multiplied with such unity amplitude sineor cosine waves, the average value would be zero.





Thus to find out any specific harmonic (x th harmonic) sine wave

from a harmonic prone waveform all we have to do is to multiplythis non-sinusoidal waveform by:

1.sin(x t) AND

1.cos(x t)

i.e. waveform is f( t)

Then find out x th term of harmonic present

f( t) X sin (x t) = b x/2f( t) X cos (x t)= a x/2

This would give overall coefficient e x2 = a x

2 + b x2.





Needless to say that if the waveform is multiplied by fundamental sineand cosine waves with unity amplitude, we can even get thefundamental part of the waveform.i.e. we can get e1 ± fundamental combined Fourier coefficient.

This would give us RMS value of fundamental sine wave.

Subtracting this from harmonic prone waveform overall RMS value, wecan get the overall value of other harmonic terms combined together.

Thus, now the THD-F and THD-R can easily be derived.

Where THD-F = Harmonics RMS value / Fundamental RMS valueTHD-R = Harmonics RMS value / Total waveform RMS value.




Tushar Mogre·s Power & Energy Consultancy Page: 432 Day seminar Power Factor ± Electrical Supply Harmonics± P Q lit ff t d t th

Any Questions ?

Thank You,End of Day 2 ±

Session first ½

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PF Harmonics Effect+on+PQ +Day2+First+Half for+Printing