Focus: Power Factor Correction

Baldev Raj Narang

Electrical circuits carry currents ofundesirable frequencies besidesfundamental current of 50Hz or

60Hz. Fundamental current performsthe intended work of catering to powerneeds of the loads whereas currents offrequencies which are multiples offundamental frequency create variousundesirable effects in the Electricalnetwork. When a sinusoidal voltage is

applied to a linear load it causes a sinusoidal current whilefor non linear loads the current is not sinusoidal. Thedistorted current waveform can be assumed to be made upof sinusoidal waveforms of frequencies which aremultiples of fundamental frequency. Harmonics areclassified as positive sequence (1,4,7,10,13,---), negativesequence (2,5,8,11,1---)and zero sequence (3,6,9,12,1---).

How they are generated: Harmonics are generated bynon linear loads which use a part of controlled sine waveand the load current does not vary linearly with voltage. Asagainst this linear loads use complete sine wave and theload current varies linearly with the applied voltage.

What they do: Zero sequence harmonics cause currentsin neutral conductors and develop neutral voltage withrespect to earth. Negative Sequence Harmonics causereverse electromotive forces in motors and affect the motorperformance. Heating is caused by all three types ofharmonics. Harmonics set up high frequency magneticfields and induce eddy currents in conductors.

Capacitors bear the maximum burden of harmoniccurrents due to their low impedance values at higherfrequencies. Capacitors also set up parallel resonancecondition by interacting with system impedance. If theresonance condition occurs near any of the prevailingharmonic frequencies these harmonics are amplified. Someconsequences of harmonics are capacitor failures, spurioustripping of thermal relays, measurement errors, higher

losses, overstressing and shortening life of transformers,cables and other electrical equipments, erratic operation ofcontrols, change in performance characteristics ofelectrical and electronic equipments, interference withcommunication systems etc.

A major cause of concern: Parallel resonance betweenpower factor improvement capacitors and systemreactance amplifies the harmonics to alarming levels andthe adverse effects of harmonics are magnified. Resonanceis a condition with potential for heavy damage and itoccurs when the combined impedance of inductivereactance of the system and the capacitive reactance of thesystem attains maximum value. Indiscriminate use ofcapacitors leads to resonance at undesirable frequenciesand it is the major cause of failures and problems associatedwith harmonics.

Objectives of harmonic mitigation: The objectives ofmitigating harmonics are: to have all electrical equipmentsfunctioning properly with a high probability of meetingnormal life expectations; to have operational efficiencyand energy efficiency; to maintain power quality; and tomeet the harmonic current and voltage distortion limitsprescribed by international standards or by utility supplycompanies.

Methods of mitigating harmonics: Increasingeffective source impedance: Harmonic current distortioncan be reduced by increasing the effective sourceimpedance relative to individual loads. This is commonlyaccomplished by adding a series line reactor at the input tovarious loads such as VFDs.

Harmonic cancellation: A 12-pulse converter uses two6-pulse bridge rectifiers that are supplied from twodifferent power sources. These power sources are phaseshifted by 30 electrical degrees, resulting in cancellation ofthe 5th and 7th harmonics. Similarly, the 18-pulseconverter uses three sets of 6-pulse bridge rectifiers that aresupplied from three different power sources, each of whichare phase shifted by 20 electrical degrees. This arrangementresults in cancellation of the 5th, 7th, 11th, and 13thharmonics.

Harmonics in Electrical Power Systems

44 Electrical Monitor June 2015

Comparison of Harmonic Resonance Solutions

44-46] Focus Clariant.qxp 6/8/2015 7:21 PM Page 44

Technical Insight

Active Harmonic Filter using IGBTs: AHF analyses thecurrent harmonics in the load side and generates exactlythese harmonics in 180-degree phase opposition to cancelout the harmonics. As a result, harmonic currents aresupplied from the AHF and fundamental current issupplied from the mains. If the total harmonic currentrequirement is higher than the capacity of the AHF then itmerely limits the harmonics by doing partial correctionand leaves out some of the harmonic currents to be drawnfrom the supply.

Diverting harmonics to an alternate path: Detunedand tuned filters when used in parallel with the loads allowa partial to almost full magnitude of harmonic frequenciesto be diverted to them instead of flowing through supplysystem. This is by far the most effective and mosteconomical and commonly used method of harmonicmitigation.

Detuned Filter: A detuned filter avoids harmonicamplification with normal filtration. A suitably designedreactor connected in series with the power factorcorrection capacitor is designed to have a series resonancefrequency lower than the most significant lowest orderharmonic present in the system. This is usually the fifthharmonic order harmonic having frequency of 250 Hz for50 Hz systems. The net combined impedance of LCcombination and system impedance above selectedresonance frequency is always inductive and there is nochance of resonance occurrence above this point. However,there is a chance of resonance occurrence at a lowerfrequency and it may be close to third harmonic but it doesnot cause any problem as third harmonic being a zerosequence harmonic needs neutral to flow and thecapacitors are normally connected in delta.

The LC filter combination serves the dual purpose ofshifting resonance to a safe point thus avoiding harmonicamplification and provides an inductive path to higherlevel frequencies to flow through it. A 7 per cent reactor isthe most commonly used Reactor. Its impedance is 7 percent of the impedance of the series connected Capacitor atfundamental frequency. The tuning frequency of LC seriescombination is 189 Hz. This LC series combination avoids

amplification of harmonics and provides a nominalfiltration too. The purpose of this detuned bank is not tosignificantly reduce harmonic distortion but rather toensure that the capacitor bank does not resonate with thenetwork impedance. If compliance with harmonic qualityregulations is required, alternative measures offeringhigher filtration capacity are necessary.

Single Tuned Filter: Single tuned filter avoidsharmonic amplification with partial filtration. The tuningfrequency is 210 Hz or higher. Besides avoiding harmonicamplification it is designed to absorb a significant portionof harmonics. This design usually allows harmonic levelsto be brought within IEEE 519 prescribed limits.

Multi Tuned Filter: Multi-tuned filter avoids harmonicamplification with near total filtration. When more thanone type of harmonics are significant in the system say 5thand 7th then for each such harmonic an LC series filtercombination arm tuned for individual harmonic frequency,here 5th and 7th harmonic, is connected in parallel withthe load. This allows a larger portion of harmonics to beabsorbed by filters. The resultant harmonic level in thesystem can be brought down to more stringently designedvalues. Special care in designing has to be taken here toaccount for any amplification of 5th harmonic caused bythe filter arm designed for filtering 7th harmonic.

Tapped Reactors: It is advisable to use multi tappedreactors instead of single tap construction. This allows finetuning at site as per actual working conditions andparameters, it also allows retuning under changed loadconfiguration or changed system configuration.

Selection of Capacitors and Reactors: The LC seriescapacitor-reactor combination provides an inductive pathto harmonic frequencies through them thus diverting theseharmonics away from the utility distribution system. Thisrequires the capacitors and reactors to be capable ofwithstanding increased current loadings withoutdeterioration or failure.

Reactors: Reactors dampen effect of transients duringcapacitor switching, avoid resonance and harmonicamplification and are main constituents of harmonicfiltration circuit. A hitherto common practice of using 7

Electrical Monitor June 2015 45

current waveforms with and without filter

44-46] Focus Clariant.qxp 6/8/2015 7:21 PM Page 45

Technical Insight

per cent light duty reactors which were mainly meant toprovide some protection to capacitors may have to be doneaway with in favor of appropriately tuned heavy dutyreactors which can provide filtration to achieve desiredharmonic levels.

The most critical components: Reliability and safetyunder hostile harmonic environment is the mostimportant requirement for capacitors today. The IECstandards require capacitors to be capable of continuouslyhandling 130 per cent of rated current but in harmonicenvironment this value can be exceeded and values higherthan 200 per cent can only be considered safe. This is nowbecoming possible with the advent of segmented MPP typelow ESR capacitors which have excellent over currenthandling capabilities. ESR value assumes highersignificance in view of high frequency harmonics beinghandled by capacitors. ESR value had a limited significancewhen fundamental frequency current was the onlyconsideration. In a harmonic environment higher ESRvalues lead to significantly higher levels of heating.

Reliability of capacitors in stressful harmonicenvironment, overload conditions and high operatingtemperatures is of prime importance for uninterruptedoperation of harmonic filtration and power factorcorrection systems. The segmented film MPP typeconstruction provides enhanced safety and increasesservice life of capacitor by clearing multiple faultsoccurring in close proximity within the segment before theheat and pressure developed can cause total isolationthrough overpressure interrupter. The hostile conditionscall for capacitors to have built in over pressureinterruption mechanism for safely isolating them duringoverloads and during end of their service life withoutdestroying them in an unsafe way to save the installationand surroundings from any damages. Here MPP capacitorswith continuous technological innovations anddevelopments perfectly fit the bill. MPP capacitors withworking life of 200,000 hours are available today.

Problems with APP Capacitors: There are typicalproblems associated with APP capacitors—absence ofinherent safety features; high dissipation factor; highmanufacturing tolerances; poor quality of bimetallic endconnections; abnormally high operating temperatures;loose winding with trapped moisture and impurities; lowpower density leading to higher space requirements; andhigher CO2 emission requirements due to higher losses.

The absence of any inbuilt safety mechanism to clearinternal faults leads to continued operation underabnormal conditions of over temperature, over pressureand degradation of oil, thus leading ultimately to completefailure under unsafe conditions. The undesirable mode offailure under heavy loads, or at the end of service life hasmade APP capacitors unusable worldwide for LTapplications. The ongoing research for development of HTcapacitors with a superior and safer technology may soonphase out APP capacitors for HT applications too.

Harmonic mitigation scheme: The technical benefitsof applying harmonic solutions are available upwards of

the point of application. Most solutions today are appliedby users close to the point of utility company’s supply tomeet mainly statutory requirements. The harmonicsolutions need to be shifted downwards closer to theharmonic generating loads in order to pass on the benefitsto both the utility company and the user.

Conclusion: There is increasing presence of non linearloads leading to harmonic generation and their furtheramplification due to resonance conditions induced bycapacitors. To safeguard their installations utilitycompanies are rightly coming out with penal tariffstructures to limit harmonics being fed to the grid.Reactive power management systems today need to focusattention not only on power factor correction but also onthe power quality aspects. Tariff structures which promotepower factor correction aspect alone and lead toindiscriminate use of plain capacitors causing largeamounts of harmonics in power systems need to bediscontinued in favor of more rational tariff structures.

46 Electrical Monitor June 2015

AABBOOUUTT TTHHEE AAUUTTHHOORR BBaallddeevv RRaajj NNaarraanngg is an Electrical Engineering Graduate from Delhi

College of Engineering, University of Delhi and is currently CEO of Pune-

based Clariant Power System Ltd, a company which in association with

FRAKO Germany, is engaged in providing solutions in reactive power

systems and power quality management to Indian and overseas

industrial customers in diverse fields. Narang has previously worked for

Indian Oil Corporation and Century Enka Ltd, among others. He can be

reached at baldevrajnarang@clariantindidia.co.in

MECO has introduced new Digital Clampmeters Model2502T-Auto and 2520THz-Auto with the following

features • Range: 1000A AC • Handy and User friendly •Auto/ Manual range selection • Non Contact Voltage testingfunction • Temperature measurement • Frequencymeasurement (2520THz-Auto) • Data Hold• Max (2502T-Auto) • Auto Power Off • Capacitance (2520THz-Auto)

2502T-Auto is a 3½ digit 2000 Counts digital clamp meterhaving current range up to 1000A AC and voltage range750VAC and 1000V DC with the jaw opening size of 52mm.Basic accuracy for AC Current is ± 2% rdg + 5dgt, for ACVoltage ± 1.2% rdg + 3dgt and for DC Voltage ±0.8%rdg +3dgt. In addition it has features like NCV, temperature, maxfunction, resistance, audible continuity, diode test functionetc. 2520THz-Auto is a 3¾ digit 4000 counts digital clampmeter having current range up to 1,000A AC and voltage range750V AC and 1,000V DC with jaw opening size of 52mm.Basic accuracy for AC current is ±2.5%rdg + 5dgt, for ACVoltage ±1.2%rdg + 3dgt and for DC voltage ±0.8%rdg +3dgt. In addition it has features like NCV, temperature,capacitance, frequency, resistance, audible continuity, dutycycle, diode test function etc.

For details please visit www.mecoinst.com

MECO Digital Clamp meters

44-46] Focus Clariant.qxp 6/8/2015 7:21 PM Page 46