Harmonics Harmonics in in Low Low Voltage Voltage Three-Phase Three-Phase Four-Wire Four-Wire Electric Electric Distribution Distribution Systems Systems and and Filtering Filtering Solutions Solutions Dr. Dr. Prasad Prasad Enjeti Enjeti Texas Texas A&M A&M University University Power Power Electronics Electronics and and Power Power Quality Quality Laboratory Laboratory PS PSERC ERC Online Online Seminar Seminar February February 13, 13, 2001 2001 ©©2001 2001Texas TexasA&M A&MUniversity. University.All Allrights rightsreserved. reserved. Outline Outline 1 Introduction Introduction 2 Harmonic Harmonic Current Current Sources Sources 3 Case Case Studies Studies 4 Solutions: Solutions: -- Derating Derating -- Passive Passive Approach Approach -- Active Active Approach Approach 5 Conclusions Conclusions Introduction Typical Typical Building Building Power Power Distribution Distribution Scheme Scheme Harmonic Current Sources Harmonic Harmonic Producing Producing Equipment Equipment in in Office Office Buildings Buildings Copy machines Recorders Electric typewriters Televisions Light fixture ballasts Video tape players Personal computers SCR drives for motors Computer systems SCR drives for elevators Computer terminals UPS systems Audio visual equipment Laboratory testing equipment Harmonic Harmonic Related Related Problems Problems in in Buildings Buildings •Overheating and damage to neutral conductors •Overheating and damage to panel board feeders •Line voltage distortion •Higher Common mode voltage •Nuisance tripping of circuit breakers •Overheating and premature failure of distribution transformers Case Studies Typical Typical Current Current Waveforms Waveforms of of Appliances Appliances Typical Typical Current Current Waveforms Waveforms of of Appliances. Appliances. Contd. Contd. Typical Typical Current Current Waveforms Waveforms of of Appliances. Appliances. Contd. Contd. Typical Typical Current Current Waveforms Waveforms of of Appliances. Appliances. Contd. Contd. Effect Effect of of Triplen Triplen Harmonics Harmonics in in Neutral Neutral Conductors Conductors Ia Ib I N ,rms = I a2,rms + I b2,rms + I c2,rms I N ,rms = 3 * I phase ,rms Ic IN Survey Survey results, results, Building Building II Survey Survey results, results, Building Building II II Summary Summary of of Building Building Measurement Measurement Phase Building A B C N Neutral to Phase, % Building I 66.8A 77.3A 72.5A 120.0A 170% Building II 50.0A 65.6A 41.1A 53.7A 103% Building III 12.8A 36.1A 24.5A 27.5A 80.9% Building IV 80.6A 77.2A 90.0A 142.0A 171.3% Building V 23.0A 17.4A 26.0A 12.7A 57.4% Building VI 44.6A 41.2A 41.9A 60.7A 143% Harmonic Harmonic Profiles Profiles for for Typical Typical Buildings Buildings OFFICE OFFICE BUILDINGS BUILDINGS MEDICAL MEDICAL FACILITIES FACILITIES • 9 Buildings surveyed. • Typical loads include computers and office equipment. • Average neutral to phase ratio of 1.13 • Average crest factor of 2.12 • 22 facilities surveyed. • Typical loads include computers, office equipment, diagnostic and patient care equipment. • Average crest factor of 2.02 INDUSTRIAL INDUSTRIAL FACILITIES FACILITIES • 14 sites surveyed. • Typical loads include computers, laboratory equipment, telecommunication equipment. • Average neutral to phase of 1.04 • Average crest factor of 2.12 Harmonic Harmonic Profiles Profiles for for Typical Typical Buildings. Buildings. Contd. Contd. GOVERNMENT GOVERNMENT BUILDINGS BUILDINGS • 6 sites surveyed. • Typical loads include computers, data processing equipment, office equipment. • Average neutral to phase of 0.77 • Average crest of 1.78 AUDIO AUDIO VISUAL VISUAL STUDIOS STUDIOS •8 studios surveyed. •Average neutral to phase of 0.65 •Average crest of 1.74 Common Common Mode Mode Noise, Noise, Building Building II Line Line Voltage Voltage Distortion, Distortion, Building Building II Harmonic Harmonic Effects Effects on on Transformers Transformers TRANSFORMER TRANSFORMER LOSSES LOSSES DUE DUE TO TO TRIPLEN TRIPLEN HARMONICS HARMONICS EFFECTS EFFECTS OF OF TRIPLEN TRIPLEN HARMONICS HARMONICS • Losses in transformers are subdivided into core and winding losses. • Excessive stress due to heating • Core loss is of minor concern since it is due to flux generated in the core by the bus voltage. • Winding losses are increased due to I2R and stray losses. • Insulation breakdown • Lower operating efficiency • Short life span • Acoustic noise Solutions: Solutions: -Derating -Derating -Passive -Passive Approach Approach -Active -Active Approach Approach Recommendations Recommendations by by CBEMA CBEMA 1. Derate transformers or use K-Factor Transformers. 2. Oversize all neutral components for 1.73 times rated full load amps. 3. Use separate neutral conductors for nonlinear loads and avoid shared neutral conductors where practical. 4. Use neutral over current sensors to trip phase conductors. 5. Use true rms. ammeters and instruments with sufficient bandwidth for measurement. Transformer Transformer Derating Derating Schemes Schemes Derating is a means of determining the maximum load that may be safely placed on a transformer that supplies harmonic loads. The most common derating method is the CBEMA approved "crest factor" method which provides a transformer harmonic derating factor,THDF. Example Example of of Transformer Transformer Derating. Derating. Building Building II Phase True RMS Phase Current Peak Value of Phase Current A 70A 178A B 76A 181A C 73A 180A HENCE, THE TRANSFORMER SHOULD BE DERATED TO 57% OF ITS NAME PLATE RATING!!! KK Factor Factor Rated Rated Transformers Transformers •K factor was developed by Underwriters Laboratory in UL 1561. •K factor transformers are designed to supply nonsinusoial loads. •They contain enlarged primary windings to carry circulating triplen harmonic currents. •The magnetic core has a lower flux density as it is designed with higher grades of iron. •K factor transformers use smaller, insulated, secondary conductors in parallel to reduce skin effect. •K factor transformers are more expensive than conventional transformers. KK Factor Factor Rated Rated Transformers. Transformers. Contd. Contd. K factor is a means of rating a transformer with respect to the harmonic magnitude and frequency of the load. The K factor numbers do not linearly indicate transformer harmonic tolerance. For example, a K4 rated transformer has four times the eddy current tolerance as a K1 transformer. A K13 rated transformer has approximately twice the tolerance of a K4 and K30 has twice of a K13. Example Example KK Factor Factor Calculation. Calculation. Building Building II K K Factor Factor == 8.84 8.84 Solutions. Solutions. Neutral Neutral Cable Cable Super Super Neutral Neutral Cable® Cable® Super SuperNeutral NeutralCable Cableisisaametal-clad, metal-clad,Type TypeMC MCCable, Cable, manufactured manufacturedwith withan anoversized oversizedneutral neutralconductor conductoror orone one neutral per phase for three-phase/four-wire power supply neutral per phase for three-phase/four-wire power supply systems systemsto tocomputers computers(with (withdc dcdrive drivefan fanmotors, motors,tape tapeand and disk diskdrives) drives)office officemachines, machines,programmable programmablecontrollers, controllers,and and similar similarelectronic electronicequipment equipmentwhere wherenon-linear non-linearswitching switching loads loadsproduce produceadditive, additive,odd oddorder orderharmonic harmoniccurrents currentswhich which may create overloaded neutral conductors. may create overloaded neutral conductors. The Theoversized oversizedneutral neutralconductor(s) conductor(s)are aresized sized150% 150%to to200% 200% of ofthe thephase phaseconductor conductorampacity ampacityto tominimize minimizethe theeffects effectsof of harmonics harmonicsgenerated generatedby bythe thenon-linear non-linearloads. loads. The Theneutral neutral per phase (striped with color to match the phase conductor) per phase (striped with color to match the phase conductor) accomplishes accomplishesthe thesame sameobjective. objective. Super SuperNeutral NeutralCable Cablecan canbe beused usedunder undercomputer computerroom room floors, floors,raised raisedfloors, floors,or oroverhead overheadin inthe thespace spaceabove abovehung hung ceilings ceilingsused usedfor forenvironmental environmentalair airhandling. handling. ItIthandles handles branch and feeder, plus power, lighting, signal, and branch and feeder, plus power, lighting, signal, andcontrol control circuits circuitsin indry drylocations. locations. Solutions. Solutions. Transformers Transformers K-factor transformers: Sola Dongan Disadvantages Disadvantages and and Risks Risks Derating a transformer is a temporary fix and often translates into lower efficiency operation and increased heat for losses. Derated transformers also run the risk of being perceived to be partially loaded and future load additions are possible. While derating removes some stresses from the transformer, a typical dry type transformer is not designed to supply harmonic loads. Hence, it may be subject to a shorter life span and lower efficiency. Oversizing of transformers, or selection of unnecessarily high K factor ratings of transformers, can increase the harmonic currents due to lower impedance. Separate Separateneutral neutralconductors conductorsfor forcomputer computerloads loadsisisalmost almostimpossible impossible to toimplement, implement,due dueto toaawide widescattering scatteringof ofdata dataprocessing processingequipment equipment all allover overthe thebuilding. building. Solutions: Solutions: -Derating -Derating -Passive -Passive Approach Approach -Active -Active Approach Approach Passive Passive Approach Approach for for Neutral Neutral Current Current Reduction Reduction Circulates Circulates excessive excessive triplen triplen harmonics harmonics through through the the filter filter and and back back to to the the load. load. Protects Protects the the distribution distribution transformer transformer and and neutral neutral conductor conductor from from excessive excessive neutral neutral currents. currents. Improves Improves transformer transformer power power factor. factor. Zig-Zag Zig-Zag Transformer Transformer for for Neutral Neutral Current Current Reduction Reduction VA VA Rating Rating Calculation Calculation T-Connected T-Connected Transformer Transformer for for Neutral Neutral Current Current Reduction Reduction VA VA Rating Rating Calculation Calculation kVA kVA rating rating is is higher higher Star-Delta Star-Delta Transformer Transformer for for Neutral Neutral Current Current Reduction Reduction Transformer Transformer kVA kVA rating rating is is higher. higher. Harmonic Harmonic filtering filtering is is impedance impedance sensitive. sensitive. VA VA Rating Rating Calculation Calculation Zig-Zag Zig-Zag Transformer Transformer for for Neutral Neutral Current Current Reduction Reduction Disadvantages Disadvantages Zero Zero sequence sequence impedance impedance of of the the Zig-Zag Zig-Zag transformer transformer must must be be low. low. This This requires requires special special design. design. rd The harmonic current current The effectiveness effectiveness of of the the Zig-Zag Zig-Zag transformer transformer to to divert divert 33rd.. harmonic is is highly highly dependent dependent on on the the distribution distribution system system impedance. impedance. In In most most cases cases only only 50% 50% reduction reduction can can be be guaranteed. guaranteed. Lower Lower zero-sequence zero-sequence impedance impedance increases increases the the single-phase single-phase fault fault current current level. level. Therefore, Therefore, fusing fusing and and circuit circuit breaker breaker re-sizing re-sizing may may be be necessary. necessary. The The specially specially designed designed low low impedance impedance Zig-Zag Zig-Zag transformer transformer becomes becomes aa low low impedance impedance path path for for zero zero sequence sequence currents currents from from other other parts parts of of the the system system Due Due to to the the reasons reasons mentioned mentioned above above the the passive passive Zig-Zag Zig-Zag transformer transformer approach approach is is larger larger in in size size and and weight. weight. Solutions. Solutions. Passive Passive Approach Approach Acme's Acme's I-Trap™ I-Trap™ Reduces Reduces Excessive Excessive Neutral Neutral Currents Currents Caused Caused by by Harmonic Harmonic Distortion Distortion Acme Acmehas hasintroduced introducedaaproduct productthat thatprovides providesaacost costeffective effectivesolution solutionto to high neutral currents that result from zero sequence harmonics and high neutral currents that result from zero sequence harmonics and unbalanced unbalancedloads. loads. The TheI-TRAP™ I-TRAP™units unitsare aredesigned designedfor foruse useon on208Y/120V 208Y/120Vthree threephase phase60Hz 60Hz systems. They provide electrical contractors, system designers and systems. They provide electrical contractors, system designers and specifying specifyingengineers engineerswith withan aneconomical economicalalternative alternativeto toreplacement replacementof of overheated system neutral conductors. The I-TRAP™ can be installed overheated system neutral conductors. The I-TRAP™ can be installed adjacent adjacentto todistribution distributionpanels panelsthat thatwill willfeed feednon-linear non-linearloads loadsin inplants, plants, offices, offices,hi-rises hi-risesand andother otherlocations. locations.This Thispermits permitsthe theuse useof ofsmaller, smaller,less less expensive neutral cable and provides protection to distribution expensive neutral cable and provides protection to distribution transformers transformersthat thatmay mayoverheat overheatdue dueto toharmonic harmonicloads. loads. The TheI-TRAP™ I-TRAP™isisfor foruse useon onsystems systemswith withmaximum maximumneutral neutralcurrents currentsof of 100A, 150A, 300A and 450A. Each unit includes a built-in ammeter to 100A, 150A, 300A and 450A. Each unit includes a built-in ammeter to provide provideconstant constantreading readingof ofthe theneutral neutralcurrent. current. Installation Installationisisaccomplished accomplishedby byfour foursimple simpleterminal terminalconnections connectionsand andthe the units can be floor or wall mounted and require minimal maintenance. units can be floor or wall mounted and require minimal maintenance. All AllI-TRAP™ I-TRAP™units unitsare areUL ULlisted listedand andCSA CSAcertified, certified,rated ratedfor for115 115deg. deg.CC temperature temperaturerise riseand and220 220deg. deg.CCinsulation insulationclass, class,and andare arecovered coveredunder under Acme's exclusive 10 year warranty. Acme's exclusive 10 year warranty. The TheI-TRAP™ I-TRAP™and andAcme's Acme'sfull fullline lineof ofdry drytype typedistribution distributiontransformers transformers are available through a network of reps and electrical distributors are available through a network of reps and electrical distributorsin inthe the United States and Canada. United States and Canada. Solutions: Solutions: -Derating -Derating -Passive -Passive Approach Approach -Active -Active Approach Approach Active Active Neutral Neutral Current Current Filtering Filtering Scheme Scheme - + Single & ThreePhase Linear and Non-linear Distributed Loads - + - + In Harmonix TM: Active Harmonic Cancellation System I ref = 0 Circulates Circulates excessive excessive triplen triplen harmonics harmonics through through the the filter filter and and back back to to the the load. load. Protects Protects the the distribution distribution transformer transformer and and neutral neutral conductor conductor from from excessive excessive neutral neutral currents. currents. Improves Improves transformer transformer power power factor. factor. Active Active Neutral Neutral Current Current Filtering Filtering Scheme Scheme •Developed by Texas A&M University, College Station & Current Technology, Inc. Dallas, Texas. •Design to Cancel Zero Sequence Harmonic Current from a 3-Phase 4-Wire Distribution System. •Patented Technology. Active Active Neutral Neutral Current Current Filtering Filtering Scheme Scheme Active Active Neutral Neutral Current Current Filtering Filtering Scheme. Scheme. Functional Functional Block Block Diagram. Diagram. a - + - + - + I N /3 b I N /3 c n S in g le & T h re e P h a s e L in e a r a n d N o n -lin e a r D is trib u te d L o a d s I N /3 In In - IN IN T1 I se n sed N S e n s in g C irc u itry 60H Z N o tc h F il t e r E rro r A m p li f i e r I ref = 0 Lf PW M C o n tro lle r 3 -P h a se B rid g e R e c tifie r P W M F u llB rid g e In v e rte r + Active Active Neutral Neutral Current Current Filter Filter Active Active Neutral Neutral Current Current Filter. Filter. Features. Features. •High Cancellation Effectiveness : 90 - 95%. •Performance independent of System Impedance. •Cancels neutral current harmonics by measurement and close-loop control. •No low-impedance path for Zero-Sequence 60 Hz. •Built-in pulse-pulse current limit (No Overloading). •Significant improvement in Voltage and Current THD’s. •Fast-response characteristics. Active Active Neutral Neutral Current Current Filter. Filter. Connection. Connection. F ilte r O n F ilte r O n / O ff I-m a x I-H a rm o n ic I-R M S A la rm D is a b le d A la rm O n / O ff H a rm o n ic F ilte r C a n c e lla tio n C u rre n t F a u lt N o rm a l T est C u r r e n t T e c h n o lo g y L ow T h e rm a l B a tte ry O v e rlo a d H a r m o n ic C o n tr o l P a n e l 4 .5 A m ps F a c ility N e u tra l C u rre n t 1 0 2 .8 A m ps H A R M O N IX A c tiv e H a r m o n ic C a n c e lla tio n S y s te m / 1 0 0 a m p A B C N 3 Phase Breaker B re a k e r Current Sensor 120/208 - 225 Amps Distribution Panel F ilte r O n F ilte r O n / O ff I-m a x I-H a rm o n ic I-R M S A la rm D is a b le d A la rm O n / O ff H a rm o n ic F ilte r C a n c e lla tio n C u rre n t F a u lt N o rm a l T est C u r r e n t T e c h n o lo g y L ow T h e rm a l B a tte ry O v e rlo a d H a r m o n ic C o n tr o l P a n e l 4 .5 A m ps F a c ility N e u tra l C u rre n t 1 0 2 .8 A m ps H A R M O N IX A c tiv e H a r m o n ic C a n c e lla tio n S y s te m / 1 0 0 a m p B re a k e r Current Resistor Term. Box N G Active Active Neutral Neutral Current Current Filter. Filter. Connection. Connection. Filter Filter Specifications Specifications Voltage 120/208, 220/380, 277/480 Current 100 A rms per module Frequency 50/60 Hz Power Efficiency 90% Cancellation Effectiveness 90-95% Parallelability Up to 4 filters can be connected in parallel Topology Parallel connection 3-Phase, 4-Wire Filter Filter Specifications. Specifications. Contd. Contd. Protection Pulse-Pulse current protection Thermal shutdown Overcurrent protection Monitoring Facility current Filter current Harmonics or total rms current Status Indication Indicator lights, Audible alarm, Max current indicator, Dry contacts Enclosure 21H x 24W x13D Floor mount stackable Weight 200 lbs (approx.) Active Active Neutral Neutral Current Current Filter. Filter. Test Test Results Results 1 2 0 /2 0 8 D is tr ib u tio n P a n e l B o a rd s L o a d C e n te r PC PG 2 S in g le - p h a s e L in e a r & N o n - L in e a r L oads PE H a r m o n ix : A c tiv e H a r m o n ic C a n c e lla tio n S y s te m TM O n e - L in e D ia g r a m - B e ta S ite E le c tr ic R o o m 1 0 3 Active Active Neutral Neutral Current Current Filter. Filter. Test Test Results Results Neutral Current without Active Harmonic Cancellation System Neutral Current with Active Harmonic Cancellation System Active Active Neutral Neutral Current Current Filter. Filter. Test Test Results Results Variable Without Filter With Filter Neutral Current (A rms) 82.1 33.0 Zero-Sequence Harmonic Current in Neutral (A rms) 71.5 4.14 60 Hz Zero-Sequence Current in Neitral (A rms) 34.3 32.7 Voltage THD (% Fundamental) 3.3 2.9 Current THD (% Fundamental) 67.2 33.3 Cancellation Effectiveness - 94.2 Neutral Neutral Current Current Filtering Filtering Scheme Scheme Conclusions Conclusions The The active active filter filter employs employs aa conventional conventional zig-zag zig-zag transformer transformer (not (not optimized optimized for for low low zero zero sequence sequence impedance) impedance) and and cancels cancels the the neutral neutral current current harmonics harmonics by by measurement measurement and and closed closed loop loop current current control. control. The The performance performance of of the the active active filter filter is is independent independent of of the the system system impedance, impedance, hence hence is is not not location location sensitive. sensitive. The The active active filter filter compensates compensates only only for for harmonic harmonic currents currents in in the the neutral neutral and and does does not not become become aa low low impedance impedance path path to to unbalanced unbalanced 60Hz 60Hz currents. currents. The The active active filter filter has has an an electronic electronic built built in in current current limit limit and and does does not not overload. overload. The The proposed proposed active active filter filter is is rugged rugged and and stable stable in in operation. operation. Notice Notice the the power power electronic electronic components components are are not not subjected subjected to to line line disturbances disturbances such such as as over over voltages, voltages, voltage voltage spikes spikes etc. etc. The The kVA kVA of of the the power power electronic electronic active active current current source source in in low low as as the the voltage voltage between between the the neutral neutral of of the the zig-zag zig-zag transformer transformer and and the the system system neutral neutral is is near near zero. zero.