PFC and Harmonics

8/7/2019 PFC and Harmonics

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Power Factor Correction in presence of harmonics

When Power Factor Correction is necessary we have to bear in mind that nowadaysmany loads generate harmonic currents that must be taken into account whendesigning the PFC system. Therefore its very important for the reliability, durability andsafety of the installation to carefully analyze the load and grid to determine the properPFC technologies to be applied.

Selecting the proper PFC technologies:

Once determined the KVAR requirements or effective power to take the load to thedesired higher PF, its time for the mentioned analysis. It should really start by analyzingthe load change speed and this will determine the first PFC technology approach.If we know the load changes within minutes, seconds or tenths of a second we shouldthink from the very beginning in a dynamic PFC system (basically a system switched bythyristor switches) while if we determine that the load will change along hours, astandard contactor switched system will be fine. Another criterion would be theexpected switchings of the PFC system. A reasonable limit to choose a technology orthe other would be 50 switchings per day. For more than this, a dynamic system isreally necessary.

Once chosen standard or dynamic switching technology its time to analyze theharmonic contents of the load, and this is properly done by taken measurements with aharmonics analyzer, but only once the normal operating conditions of the load havebeen identified and all existing capacitors have been disconnected because we need todetermine the real harmonics contents of the load, without any PFC system connected.

The harmonics analysis has to clearly identify the total voltage distortion THD-V% , thetotal current distortion THD-I% and the harmonics spectrum. This means that we haveto know which harmonics are present and how high they are.

If we determine that THD-V% is higher than 3% or THD-I% is higher than 10% aharmonics filtered PFC system will be necessary.

The effects that the harmonics can cause in an installation is dependant on thetransformer that is supplying the energy, the smaller the transformer the moresignificant the effect will become on the installation, and its also dependant on capacitorbank power to be connected for PFC purposes.


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Objectives when correcting PF in presence of harmonics:

We must have clear in mind which our objectives are when we face a PFC project inpresence of harmonics. These objectives are here below described and set in theirproper priority:

1. Correct the low Power Factor: this is the main intention of the project.And we have to do it in a reliable and safe way, even in presence of harmonics.

2. Avoid parallel resonance: this is a very dangerous phenomenon that appearswhen we put capacitors in parallel with a transformer while we have harmonics

currents. So avoiding this is definitely a restriction imposed by the conditions.

3. Partially filtering the dominant harmonic: this maybe a collateral benefit if weapply the proper filtering PFC technology.

When correcting the power factor with the proper technology it is possible to achievethese three objectives, but it must be clear that Power Factor Correction is the mainobjective and not harmonics elimination, although sometimes, some partial filtering ofthe dominant harmonics can be obtained with relatively simple Detuned PFCtechnology.

Non Linear Loads: harmonics generators.

Some electrical loads, when sinusoidal voltage is applied to them, do not generate asinusoidal current. These are called Non Linear Loads.


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The periodical distorted current that they generate is equivalent to having a sinusoidalcurrent of the fundamental frequency plus many other sinusoidal currents of frequenciesmultiple of the fundamental and of different amplitudes. This can be mathematicallydemonstrated by the Fourier Series analysis.

Some non linear loads, like a motor drive or a rectifier, generate a discrete and welldefined harmonics spectrum while some others like welders generate a changing and

random spectrum.

Indeed any of them can have low PF as well and therefore PFC must be applied evenunder these conditions, but in such cases, only using the proper PFC technology will bepossible to do it in a reliable and safe way, otherwise these totally unexpected highfrequency currents will overload the system and might cause dangerous effects.

Typical non linear loads harmonics spectrums:

A single-phase full wave rectifier generates the following harmonics spectrum:

V

I1

I5I 7

LI


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Adjustable speed drives produce highly distorted current and notching on the voltage

due to pulse rectifiers switching.

Common problems caused by harmonic currents:

Overheating of transformers and rotating equipment

Neutral overloading and unacceptable neutral-to-ground voltages Failed capacitor banks

Breakers and fuses tripping

Unreliable operation of electronic equipment Erroneous energy meters

Wasted energy and higher electricity bills (kW and KWh) Wasted power distribution capacity Higher maintenance costs of equipment and machinery

Harmonics amplification caused by standard PFC systems:

Capacitors impedance decreases as a function of the frequency and that means highfrequency harmonic currents will find a low impedance path thru the capacitors.


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Harmonic currents will flow thru the capacitors without any control and will overloadthem, perhaps beyond their admissible current overload capacity.

Capacitors along with the inductance of the transformer make a parallel resonant circuit,that if not controlled, it may increase the impedance at a frequency of an existing andconstant harmonic current therefore generating over voltage at that frequency andincreasing the current at this frequency inside the parallel this circuit. A behavior ofharmonics amplification.

Detuned PFC Filters theory:

Lets pretend we have a simple installation with a transformer supplying energy to onelinear load and to one non linear load. The circuit below shows an inductor representingthe transformer inductance LT plus the grid short-circuit inductance LSC, although for in

practice LSC, is neglectable compared to LT because usually in a LV installation theshort circuit power is imposed by that of the transformer as the grid is immenselypowerful compared to the transformer.

Lets say our non linear load generates harmonic currents, from the 5th on, and thesecurrents are constant. Although the transformer is not applying any harmonic voltage,the load generates from 5th harmonic currents on, therefore behaving like a constant

Meter: 0001 K-fac tor: 1.041 Volts : 277 Frequency:60. 01 Hz

H# % H# % T.H.D.: 2.8% max: 2.9% min: 0.5%

1 100.0 0 2 0.0 693 0.4 116 4 0.1 68

5 2.1 272 6 0.0 69

7 1.6 41 8 0.0 70

9 0.2 133 10 0.0 68

11 0.4 11 12 0.0 68

13 0.7 36 14 0.1 68

15 0.1 68 16 0.0 68

17 0.2 37 18 0.0 158

19 0.1 69 20 0.0 158

21 0.1 327 22 0.0 69

23 0.0 69 24 0.0 69

25 0.1 301 26 0.0 8

27 0.0 158 28 0.0 248

29 0.1 319 30 0.0 309

31 0.2 20 32 0.0 68

Meter: 0001 K-fac tor: 1.829 Volts: 290 Frequency:59 .97 Hz

H# % H# % T.H.D.: 18.8% max: 21.6% min: 1.9%

1 100.0 0 2 0.1 263 0.5 352 4 0.3 31

5 18.8 203 6 0.0 259

7 1.2 126 8 0.0 259

9 0.0 80 10 0.0 200

11 0.1 312 12 0.0 259

13 0.0 80 14 0.0 80

15 0.1 116 16 0.0 200

17 0.0 320 18 0.0 169

19 0.0 319 20 0.0 259

21 0.1 192 22 0.0 259

23 0.0 169 24 0.0 349

25 0.1 259 26 0.0 259

27 0.1 259 28 0.0 349

29 0.0 259 30 0.0 259

31 0.1 31 32 0.0 79

No PFC With PFC

Meter: 0001 K-factor : 1.533 Amps : 1716 Frequency:60.01 Hz

H# % H# % T.H.D.: 13.6% max: 18.1% min: 2.1%

1 100.0 0 2 0.5 1003 0.4 29 4 0.1 119

5 12.3 53 6 0.4 66

7 5.5 356 8 0.1 91

9 0.7 299 10 0.2 29

11 1.3 7 12 0.1 29

13 0.1 210 14 0.0 29

15 0.3 29 16 0.0 11917 0.3 285 18 0.0 29

19 0.0 210 20 0.1 90

21 0.1 90 22 0.0 29

23 0.0 210 24 0.0 119

25 0.1 29 26 0.1 29

27 0.0 29 28 0.1 119

29 0.1 29 30 0.1 119

31 0.0 29 32 0.0 209

Meter: 0001 K-factor : 32.38 Amps: 2033 Frequency: 59.97 Hz

H# % H# % T.H.D.: 89.5% max: 152.3% min: 3.6%

1 100.0 0 2 2.3 93 1.0 169 4 2.9 79

5 150.0 263 6 3.8 259

7 8.7 141 8 1.2 300

9 1.5 280 10 1.5 259

11 1.6 259 12 0.8 310

13 1.7 279 14 0.5 259

15 1.2 79 16 1.1 29417 0.5 260 18 0.3 8

19 1.0 331 20 0.5 259

21 1.5 259 22 0.5 312

23 1.2 339 24 0.1 259

25 0.5 180 26 0.6 349

27 0.9 182 28 0.3 307

29 0.7 349 30 0.2 19

31 0.6 292 32 0.0 259


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current generator, and so its represented in graphics. Lets say in this example that the5th is the highest harmonic, the dominant.

These undesirable harmonic currents flow thru whatever is connected in parallel closingthe circuit until they return to the generator and on their way overloading everything thatcomes across.

Lets say this simple installation has low power factor so we have to install a capacitor inparallel to correct it. In order to understand what would happen, lets analyze the

impedance as a function of the frequency between points AO towards the transformer.We have the impedance of the transformer represented by a positive slope straight linemeaning that its impedance grows as the frequency grows.On the frequency axis we mark some particular points, first the fundamental frequencyf1 and then f5, the 5th harmonics, where the existing harmonic currents spectrum starts.Between f1 and f5 we have a frequency range with no existing harmonic currents.

Now when we install a capacitor for PFC, a parallel resonant circuit is formed betweenthe transformer inductance and the capacitors capacitance. This parallel circuit has a

resonance frequency for which its impedance becomes very high, and this parallelresonance frequency is determined by the values of the inductance and capacitanceaccording to this formulae:

Lsc+LT

loads

In

A

O f

IZI

f1

Impedancewithout

capacitors

f5 Existing hamoniccurrents spectrum


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In the following graphics we now also see represented the new impedance curve of thisresonant circuit, showing the parallel resonance frequency fp of high impedance fallingclose to the 5th harmonics. So the 5th harmonic current which was constant now findshigh impedance at that frequency and therefore generates a considerable additionalvoltage at that frequency. V5 = ZAO5 . I5

V5 = ZAO5 . I5If I5 is constant and ZAO5 has a considerable value, V5 is significant.This is a voltage that will be overlapped to the fundamental voltage creating a highertotal peak voltage that stresses all isolations in the installation.

This parallel resonance phenomenon can be exited even by small currents of afrequency close to that of the parallel resonant frequency determined by the values of Land C. If this happens, a small harmonic current generated by the load may cause a

high current flow between the transformer and the capacitors overloading both of themand generating over voltage for everything connected in parallel.

So far we have corrected the power factor at the fundamental by connecting but wehave created a very dangerous parallel resonant risk.

To avoid this we have top modify this parallel circuit in such a way that its parallelresonance frequency falls somewhere between the fundamental and the first existing

f = 2 ( LSC+LT ) C

1

= 2 LT C

1

Lsc+LT

C

loads

In

A

O f

IZI

fpf1

Impedancewith

capacitors

Impedancewithout

capacitors

f5 Existing hamoniccurrents spectrum


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harmonic current, in this example, between f1 and f5, a frequency range where weknow that there are no currents.

The circuit is modified by adding a reactor in series with the capacitor with aninductance value selected in such a way to get the desired effect.

The now more complex circuit shifts the parallel resonance frequency to a safe placewhere theres no harmonic current while the reactor-capacitor set, at the fundamental

frequency, still behaves almost as if the reactor doesnt exist.

But this new reactor-capacitor set makes another new resonant circuit, now series,which creates another new series resonance frequency, fs, which falls just below thefirst existing harmonic current (in this example the 5 th). At fs the impedance of this LCset is almost zero, creating short-circuit path for a current of that frequency. This iswhats called a Tuned Filter, exactly tuned at the frequency which has to be bypassedfrom the rest the installation. But tuned filters, as they have zero impedance, have notheoretical limit to the current at fs frequency except for the practical limitation imposedby their components current transportation capability.So if our non linear load sends 100 Amps of 5 th harmonics it will flow across it (ifcomponents resist it) and it will filtered out of the rest of the installation, but it would bedangerous if theres no control of the incoming harmonics because the filter is alreadyheavily loaded with the fundamental current.

LT

CIn

A

O

LR

f

IZI

fp fsf1

Impedancewith

capacitors

Impedancewithout

capacitors

Impedancewith

Detuned Filter

loadsf5

Existing hamoniccurrents spectrum

New fp = 2 ( LT+LR ) C

1


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By selecting the components values in such a way that fs falls a little below thefrequency of first dominant harmonics, the impedance of the filter is not zero but has acertain low value at the first dominant harmonic current and therefore this impedance

limits the amount of this harmonics that flows into the filter. The impedance at this firstharmonic is controlled by detuning the filter, the closer it gets to the harmonic frequencythe more current if filters. Detuned filters are detuned in such a way that the harmoniccurrents that flow into the filter do no exceed the maximum permissible current of thecapacitor and of the reactor that make up the filter, considering the fundamental currentused for PFC purposes.

Using Detuned PFC Filters, the three objectives for reliable and safe PFC in presenceof harmonics are achieved. At the fundamental frequency, it corrects the PF, the parallelresonance is avoiding by shifting the resonance frequency to a value where theres nocurrent of that frequency and the first dominant harmonic current can be partially filtered

out from the rest of the installation without overloading the capacitor bank.

Parallel resonance risk analysis:

For a given transformer and capacitor bank combination, the parallel resonance ordercan be determined based on more know parameters like the short-circuit power of thetransformer and the capacitor bank power, according to the following formula:

Once identified the existing harmonic currents thru the harmonic analysis its easy tocheck for those existing harmonic orders which capacitor bank powers might causeresonance.

For example:

Lets suppose we have an installation with low PF and we are going to connect a PFCbank, so after taking measurements, we identify the 5th, 7th and 11th harmonic currents.The transformer has a rated power of 1000kVAR and uk%=4%

We should check for every existing harmonic order which capacitor bank output powerwould generate resonance. If the planned system to be installed might have a stepcombination that could provide any of such power ratings, detuned technology would be

n = QcSsc


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mandatory to avoid parallel resonance, even if the harmonic current at that resonancefrequency were not relevant in the measurements.

Selecting the proper detuning:

As explained, the proper detuned PFC Filter must be selected based on effective powerrequired at the fundamental and on the first dominant harmonic current found in theharmonics analysis to be partially filtered.

Detuned filters are identified for the detune factor, a parameter that identifies the seriesresonance frequency the filter is closely tuned below. Most common detune factors are7% for systems with dominant 5th harmonic or higher, and 14% for systems with

dominant 3rd harmonics.

Usually in industrial installations where control of high power 3-phase loads, the 5 thharmonic is the dominant while in commercial installations like office buildings wherelighting is the main load, the dominant harmonic is the 3rd and therefore 14% systemsare the most suitable.

Selecting the components:

The reactors are selected according to the effective power to get at rated voltage andfundamental frequency, and according to the required detune factor.

Capacitors are selected taken into account the over voltage generated by the detuningand according to matching reactor to get the effective power when in series connected.

EPCOS provides tables included in its main PFC catalogue for selecting all thenecessary components: reactors, capacitors, contactors, fuses and cables.

25000Qc =

n2Ssc

= 52 =1000 kVAr

5

25000Qc =

n2Ssc

= 72 =510 kVAr

7

25000Qc =

n2Ssc

= 112 =207 kVAr

11

Example: Xmer S = 1000 kVA uk%= 4% Ssc=25000


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Panel assembly recommendations:

Detuned PFC filters use power reactors, these are components which dissipate quiteheat and therefore operate at high temperature, about 100C.

Capacitors and contactors are components that should operate at much lowertemperature, so components placement inside the cabinet and forced ventilation are amust for reliable, durable and safe operation of the system.

Example to get: Qe = 50 kVAr - 400V - 50Hz - 7 %

UC = UN . 100100-p

= 400 V . 100

100-7

= 430 V a 440 V capacitor must be used.

Qc =100

UC . Qe =UN

2

. 1 -

100

440 . 50 kVAr= 56 kVAr (440V-50Hz)

2

.1 -

400

Calculation of the capacitor rated power Qc especified at UC = 440V,to get an effective power of 50 kVAr at 400V along with a 7% reactor

The reactor must be selected according to the effective power required at 400Vand for p=7% , therefore it will be a 50kVAr-400V-50Hz-7%Although the grid has 480V, due to the detunig the voltage on the capacitor will be:


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Mount capacitors on profiles or on separate mounting plate without thermal conduction

from the reactor and in different vertical lines to avoid heat transmission .

Mount reactors on profiles to allow cooling air flow over the reactor core.

Use abundant forced ventilation.

Connect reactors thermal-switch in series with the contactors coil circuit.


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For more information on EPCOS PFC products and technologies visit:

www.epcos.com/pfc

Author:Ricardo GarridoEPCOS AG Munich, Germany.PFC Capacitors Marketing Manager August 2006

PFC and Harmonics

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