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Power System Harmonics and Power Factor Correction

Published Jan 13, 2014 in Business & Management
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IDC EngineeringPower System Harmonics and Power Factor Correction Julie E. VanDyne, P.E.IDCEngineering, Inc.

TodaySection I - Power Factor Correction and Types of capacitor cellsMethods usedFixed capacitorsSwitched capacitors“Transient free” technologiesFuture MethodsSection II - Harmonic distortionWhat harmonics areWhere harmonics come fromEffects of harmonicsSection III - Harmonic MitigationDesign of filtersIDCEngineering, Inc.

HousekeepingWe are going to be discussing engineering topics in depthInstitute a “Buddy system”Buddy loses the “Will to Live”…Starts eyeing sharp objects…Feigns death…IDCEngineering, Inc.

Section I Power Factor Correction IDCEngineering, Inc.

Power Factor CorrectionTraditionally, we recognize three kinds of loads (electrical equipment) in industry:Resistive LoadsInductive LoadsCapacitive LoadsToday we recognize a fourthNon-Linear LoadsLeads to a discussion of power system harmonicsIDCEngineering, Inc.

Power Factor CorrectionConcentrating on the three traditional loads:ResistiveExamples – Resistance space heating, Heat tracing, infrared lamps (non-controlled)Require only Watts to functionInductive Examples – Transformers, Motors, Magnetic lighting BallastsRequire both Watts and Vars to functionCapacitiveExamples – Synchronous motors (can be), CapacitorsSynchronous motors use watts, produce Vars (if set to)Capacitors produce varsIDCEngineering, Inc.

Graphical RepresentationIDCEngineering, Inc.

Add a Var “Source”IDCEngineering, Inc.

Power TriangleTrigonometric model of Volt-amp/Watt/Var relationshipAllows use of “simple” triangle formulas to relate the threeWhere  = 0; Power Factor = 1.0, or “Unity”Watts and Volt-amps EqualIDCEngineering, Inc.

BenefitsCorrected Power Factor Same “Power” or Work performed, less current through the power delivery systemUtilities often charged rates based on KVABenefit to customer to trend toward “unity” or 100% power factorProblem:Var Loads (Motors) variedVar Sources (Capacitors) typically FixedExcessive Vars caused over-voltage conditions (reverse voltage drop due to flow of reactive current toward source)IDCEngineering, Inc.

Utility SolutionMedium VoltageSwitched with oil switchesTime clock and/or current flow basedLimited switching to around once dailyOil switches have limited lifeVacuum technology has improved this somewhat, but still not close to low voltage contactor lifeBanks/stages tended to be large – created switching transientsIDCEngineering, Inc.

Customer SolutionsPrior to around 1980Methodology – Install Capacitors WITH Motors, on load side of starter (10/25/50 Hp and larger)Many MCC’s came with Capacitor CubiclesNEC includes discussion on sizingUsually installed small fixed bank at main for remainderThis method led to maintenance headachesEach capacitor has fuses – keeping track of fuse replacement was difficultWe also learned later that it also created a lack of resonance control – harmonic problemsIDCEngineering, Inc.

Development of Controlled Capacitors – Low VoltageAround 1980, Bosch developed and imported through Var+Control of Michigan the first cheap (?) automatic controller for low voltage applicationsCame into widespread use in the 90’s, many others developed their own controller (Square D, Siemens, ABB)Most worked essentially the same way:Sensed Current on one phase, with a current transformer on phase A, and voltage phase to phase – usually phase B to C.Calculated power factor, and switched in the requisite number of steps to achieve a “target” power factor.IDCEngineering, Inc.

Developed Capacitor “Banks”Single point of power factor correction at mainCapacitance varied to correspond to the needReduced or eliminated var flow to that pointAllowed for harmonic controlUtilized traditional contactors for switchingIDCEngineering, Inc.

Electronic SwitchingLate 90’s, Israeli company ELSPEC developed low(er) cost low voltage high speed capacitor bank.Switching modulesHigh speed controllerUtilized traditional delta connected capacitorsEmployed “zero voltage” switchingOne phase always energized, the other two phases switch in as the voltage between that phase and the switched phase is zero, not necessarily zero with respect to ground.Allowed more frequent switching, very fast response (2 to 3 cycles)Applications as a low voltage static var compensatorIDCEngineering, Inc.

Similar Device - Square DSquare D originally offered banks using ELSPEC componentsVery expensive OEM partsDeveloped own switching moduleUses zero voltage switchingCompatible with standard controllerMore cost effective deviceTransient free unit ~ same cost as contactor based filter unitOrphaned unitsSupported by neither ELSPEC nor Square DIDCEngineering, Inc.

Another TechnologyStatic Var CompensationGoal was to switch the var source in and out based on voltage levelOriginally accomplished with synchronous condenser (synchronous motor)ABB/Trench developed 480 volt capacitor based unitVERY expensiveUsed special wye connected capacitors (480 volt three phase capacitors are typically delta connected)Employed zero-cross switchingTypically used for voltage control where absolutely necessaryIDCEngineering, Inc.

Future of Power Factor CorrectionSeveral companies worked toward “active filtering”, including Siemens Around 1997, Oregon company EPS developed the “Accusine”IDCEngineering, Inc.

Accusine OperationBlock diagram is that of a drive front end.Instead of SCR/Diodes, it has switching transistorsSwitches phase against phase to correct power factor and harmonicsOperates within one cycle (compared to 1.5 to3 cycles for a SVC or Square D “Real Time” unit.Presently, the technology is around 2 to 3 times more expensive than traditional capacitors, but should come down as the technology improvesIDCEngineering, Inc.

End of Section IBreak

Section II - HarmonicsWith “harmonics,” we often find ourselves talking about “Power Quality”In the field of Power Quality, we often are really talking about Voltage qualityGoal is to provide good Voltage quality to plant equipmentMagnitudeFrequencyWave shapeDistortion of the voltage wave shape is caused by the presence of harmonic CurrentsIDCEngineering, Inc.

HarmonicsPrimer Fourier - French Mathematician Describe any wave shape as the sum of a fundamental sinusoidal wave and an infinite series of harmonic sinusoidal wavesIDCEngineering, Inc.

HarmonicsHarmonicsDefine what they areHow they come about in Power SystemsCharacteristics of Individual HarmonicsMotor Drive SystemsEffects on power systems - ConsequencesIDCEngineering, Inc.

HarmonicsFundamental IDCEngineering, Inc.United States Frequency: f = 60 Hz

World System Voltage/Frequency, Inc.

HarmonicsFundamental and Third HarmonicIDCEngineering, Inc.Third Harmonic Frequency: f = 3 x 60 or 180 Hz

HarmonicsFundamental and Fifth HarmonicIDCEngineering, Inc.Fifth Harmonic Frequency: f = 5 x 60 or 300 Hz

HarmonicsFundamental and Seventh HarmonicIDCEngineering, Inc.Seventh Harmonic Frequency: f = 7 x 60 or 420 Hz

HarmonicsLinear Load  Linear Voltage DropNo Wave shape DistortionNon Linear Loads  Non Linear Voltage DropWave shape DistortionIDCEngineering, Inc.

HarmonicsLinearNon - LinearLinear vs. Non Linear LoadsIDCEngineering, Inc.

HarmonicsSwitching Mode Power Supply IDCEngineering, Inc.

HarmonicsLinear Vs. Non-LinearLoad (I) and Source (V)(Combined System Impedance (Z))Ohms LawE = I * ZIDCEngineering, Inc.

HarmonicsLinearNon - LinearLinear vs. Non Linear Voltage DropIDCEngineering, Inc.

Harmonic DistortionDistorted Voltage Waveform - Voltage Drop at the Third HarmonicIDCEngineering, Inc.

HarmonicsImplications of Individual Harmonics & Effects on Power System Need to further refine understanding of individual waveforms that make up the overall wave shape, and how they combine.IDCEngineering, Inc.

Harmonic DistortionFundamental Minus Third HarmonicIDCEngineering, Inc.

Harmonic DistortionDistorted Voltage Waveform - Voltage Drop at the Third HarmonicIDCEngineering, Inc.

Drive HarmonicsIDCEngineering, Inc.Typical drive current frequencies: 5th, 7th, 11th, and 13th

Drive HarmonicsIDCEngineering, Inc. Fundamental With Voltage Drop at the Fifth and Seventh Harmonic

Harmonic RotationFundamentalIDCEngineering, Inc.ABCBB

Harmonic RotationThird HarmonicIDCEngineering, Inc.

Harmonic RotationThird HarmonicIDCEngineering, Inc.

Harmonic RotationFifth HarmonicIDCEngineering, Inc.B

Harmonic RotationFundamental PositiveSecond NegativeThird ZeroFourth PositiveFifth NegativeSixth ZeroSeventh PositiveEighth NegativeNinth ZeroTenth PositiveEleventh NegativeTwelfth ZeroThirteenth PositiveFourteenth NegativeFifteenth Zero IDCEngineering, Inc.

ConsequencesMotorsReverse TorqueOverheating Shortened lifeTransformersIncreased LossesOverheatingShortened lifeCapacitorsIncreased CurrentOverheatingShortened lifePlant ElectronicsMiss-operationFailureDowntimeIDCEngineering, Inc.

Distortion LevelsIEEE 519-1992All Harmonics - Voltage (THD) <5% (<10%)IDCEngineering, Inc.

Distortion LevelsCurrent DistortionIDCEngineering, Inc.

HarmonicsReviewHarmonics are a way to describe the result of non-linear loads on the power systemBasic Limit of 5% (10%) for Voltage DistortionCurrent Distortion Limit Depends on System, Currently under reviewIDCEngineering, Inc.

End of Section IIBreak

Section III Harmonic Control IDCEngineering, Inc.

Source IssuesIDCEngineering, Inc.When working to deal with harmonics, we often can’t do anything with the loads.With the exception of changing from 6 to 12 pulse rectifiers, or “creating” a 12 pulse rectifier utilizing cleverly chosen isolation transformers for eachNormally, we simply work with the source

Source IssuesThevenin Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitXCIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Source IssuesNorton Equivalent CircuitIDCEngineering, Inc.

Tuning Considerations IssuesIDCEngineering, Inc.Frequency to tune to:<4.5th h – “Detuned”4.5th-5.0th h – “Filter”Popular frequencies4.2nd harmonic (252 hz)Absorbs ~50% of 5th harmonic currentModerate capacitor stressLess tuning drift4.7th or 4.8th harmonic (282-288 hz)Absorbs ~75% of 5th harmonic currentHigher capacitor stressTuning drift more of a problem

Tuning CalculationsTypical three phase capacitor - DeltaIDCEngineering, Inc. To calculate series resonance point, needs to be in a Wye configuration First, calculate line current on capacitor. Assume 100 kVAR, 480 V

Tuning CalculationsAssuming the capacitor is actually a Wye, the impedance of each capacitor (Xc) is:IDCEngineering, Inc.At 60 hz, the capacitance is therefore:Xc

Tuning CalculationsIDCEngineering, Inc.

Tuning CalculationsIDCEngineering, Inc.

Tuning CalculationsIDCEngineering, Inc.Replacing our original configuration

Tuning With CapacitorsCapacitor TolerancesIn harmonic rich environments, capacitors for the low impedance path to source currentIn filter applications, capacitors also must tolerate higher voltagesIDCEngineering, Inc.

Limitations RemainOriginal contactors were usually robustToday’s contactors limited duty, require more frequent preventative maintenance and contact replacementSomewhat slow to respond (30 seconds)Original capacitors were often VPI kraft paper design; Today’s capacitors typically use a polypropylene dielectric - less durable, less able to tolerate frequent cyclingIDCEngineering, Inc.

IEEE 18-2002 Shunt Capacitors135% Var Output110% Rated Voltage180% Rated Current90% Survival after 20 yearsIDCEngineering, Inc.

Controllers Have ProgressedControllers started as very simple devicesGraduated to more complex monitoring and diagnostic instrumentsDisplay of power quantities, Harmonic valuesOver-temperature and failure alarmsSelf-commissioningIDCEngineering, Inc.

SoftwareSpeeds calculationsProvides quality of presentationEnhances client confidenceIDCEngineering, Inc.

Summary Power factor correction today is not as important as it once was (economic issues) in utility territoriesMay become important, so stay tunedPlants in other utility areas may be affected, and worth pursuingWhich technology selected depends on the specific plant and plant environmentThe technology, as is the case with most fields today, is moving becoming better, and less expensiveIDCEngineering, Inc.

Thank YouIDC Engineering, Inc.2633 Adgate Rd.Lima, Ohio 45806P - (419) 999-4705F - (419) 991-0404jvandyne@idcengr.comwww.idcengr.comIDCEngineering, Inc.