Understanding Input Harmonics and Techniques to Mitigate Them Mahesh M. Swamy Yaskawa Electric America YASKAWA Page. 1
Organization Introduction Why FDs Generate Harmonics? Harmonic Limit Calculations per IEEE 519-1992 Harmonic Mitigation Techniques Passive and Active Means Questions and Conclusions YASKAWA Page. 2
Motivation Harmonics cause unnecessary heat in equipment connected to harmonic source System rich in harmonics is generally associated with poor power factor and low efficiency YASKAWA Page. 3
Motivation - Continued Harmonics can overload preexisting power factor correcting capacitors at plant facility and at utility distribution points Harmonics can initiate system resonance that can severely disrupt operation Hence, control of harmonic current is important and necessary YASKAWA Page. 4
Risk of Parallel Resonance Parallel Resonance X c X L i h i h Power Factor Capacitors Relieve Load Current measured at the capacitor, showing 660Hz, (11th harmonic resonance) Figure 5.2 Resonance occurs when: X c = X L YASKAWA Page. 5
Introduction Non-linear loads current does not follow applied voltage waveform YASKAWA Page. 6
Introduction (Contd.) To estimate heating effect due to non-linear currents flowing through circuit breakers and transformers, linearization is needed Resolving non-linear waveform into sinusoidal components is Harmonic Analysis Ratio of harmonic content to fundamental is defined as harmonic distortion or THD YASKAWA Page. 7
Why FDs Generate Harmonics? DC link voltage DC capacitor Phase voltage 3-phase input Rectifer input current Pulsating current due to dc bus capacitor main source of nonlinearity in input current In weak ac systems, during diode conduction ac voltage is clamped to dc bus voltage source of non-linearity in input voltage YASKAWA Page. 8
Definition of THD Ratio of the square root of the sum of squares of the rms value of harmonic component to the rms value of the fundamental component is defined as Total Harmonic Distortion (THD) If the waveform under discussion is current, then the THD definition is called Current Harmonic Distortion. If the waveform under discussion is voltage, then the THD definition is called oltage Harmonic Distortion THD I = n= n= 2 I 1 I 2 n THD = n= n= 2 1 n 2 YASKAWA Page. 9
Sample Waveforms THD = 1.2% THD = 78.3% Every Wave shape has Harmonic Distortion! YASKAWA Page. 10
Harmonic Limits Per IEEE 519-1992 Table 10.3: Current Distortion Limits for General Distribution Systems (120 through 69 k) Maximum Harmonic Current Distortion in percent of I L Individual Harmonic Order (Odd Harmonics) I SC /I L <11 11 h<17 17 h<23 23 h<35 35 h TDD <20 4.0 2.0 1.5 0.6 0.3 5.0 20<50 7.0 3.5 2.5 1.0 0.5 8.0 50<100 10.0 4.5 4.0 1.5 0.7 12.0 100<1000 12.0 5.5 5.0 2.0 1.0 15.0 >1000 15.0 7.0 6.0 2.5 1.4 20.0 Even harmonics are limited to 25% of the odd harmonic limits above. * All power generation equipment is limited to these values of current distortion, regardless of actual I sc / I L ; where I sc is the maximum short circuit current at PCC and I L is the maximum demand load current (fundamental frequency) at PCC. YASKAWA Page. 11
Harmonic Limits Per IEEE 519-1992 Table 10.2 Low-oltage System Classification and Distortion Limits Special Applications * General System Dedicated System Notch Depth 10% 20% 50% THD (oltage) 3% 5% 10% Notch Area (A N2 ) 16,400 22,800 36,500 Note: The value of A N for other than 480 systems should be multiplied by /480. * Special applications include hospitals and airports. A dedicated system is exclusively dedicated to the converter load. In volt-microseconds at rated voltage and current. YASKAWA Page. 12
Definitions PCC - Point of Common Coupling Point where harmonic measurement is to be made Typically, where the utility power comes into the business (commercial building or industrial factory) Also defined as the point where non-linear load meets the linear load within a plant most popular definition used by Consultants to enforce Drive Manufacturers to meet IEEE519 at FD input TDD Total Demand Distortion Harmonic current distortion in percent of maximum demand load current. The maximum demand current interval could be either a 15-minute or a 30-minute interval. YASKAWA Page. 13
Definitions - Continued I SC : Short-circuit current at PCC Defines the size of the customer from Utility s view point helps to distinguish between a Seven-Eleven store from a Steel manufacturing plant I L : Maximum demand load current at fundamental frequency Need not be the rated load current. YASKAWA Page. 14
Characteristic Harmonics in Rectifiers h = ( k q) ± 1 h is harmonic order, k is any integer, q is number of pulses at the dc bus voltage in one period For a six-pulse system, h will be: 5th, 7th, 11th, 13th, etc. For a twelve-pulse system, h will be: 11th, 13th, 23rd, 25th, etc. Amplitude of harmonics is 1/h for a three-phase ac to dc rectifier with no dc bus capacitor Harmonics of order other than those given above are called noncharacteristics harmonics and are more common than not YASKAWA Page. 15
Application Example for Applying IEEE519 480, 100hp FD fed from a 1500-kA transformer of 4% impedance Step 1: Identify PCC take default to be at FD terminals Step 2: Determine I SC from end user. In its absence, use transformer ka rating and percent impedance I I SC SC = = 3 ka 1000 LL (% Z 1500 1000 = 3 480 0.04 /100) 45,105 Step 3: Determine I L from user. In its absence, use NEC Amps for rated horsepower condition. Here, use 124A Step 4: Determine I SC /I L. Look up Table 10.3 to determine limit - 15% YASKAWA Page. 16
Application Example for Applying IEEE519 I SC /I L for the present case is 364. From Table 10.3, TDD < 15% of Rated Fundamental Current or 18.6A in this example; Hence IEEE 519 compliance does not mean 5% TDD If maximum demand load current is only 45A due to load condition, and harmonic distortion is 35% at this operating point, spirit of IEEE 519-1992 is still met since 35% of 45A is 15.8A, which is less than the allowable 18.6A. Don t forget that voltage distortion limits are more important than current distortion limits due to the fact that voltage is common to all customers on the same grid, while current is local to a load YASKAWA Page. 17
Harmonic Mitigation Techniques Active Techniques Passive Techniques Hybrid Techniques Combination of Active and Passive Techniques YASKAWA Page. 18
Active Mitigation Techniques Active Front End Boost Converter Topology Inherently regenerative. Bulky, and expensive. Conducted EMI is of concern Non regenerative type: Inject Current from conducting phase to non-conducting phase using semiconductor switches Shunt type: Monitors load current and injects mirror image of load current so that harmonics cancel out. YASKAWA Page. 19
Passive Mitigation Techniques AC Line Inductors (Reactors) DC Link Chokes or DC Bus Inductor Harmonic Filters Capacitor based Multi-pulse Schemes YASKAWA Page. 20
Passive Mitigation Techniques AC Line Inductors (Reactors) Makes discontinuous current continuous Helps damp transient surges on line due to lightning and capacitor switching Small and inexpensive Causes voltage overlap and reduces dc bus voltage YASKAWA Page. 21
Passive Mitigation Techniques AC Line Inductors (Reactors) THD 80%. THD 40%. YASKAWA Page. 22
YASKAWA Page. 23 Issues With AC Line Reactors DC Bus oltage Reduces Due to Overlap of Diode Conduction π μ π μ ω ω π π μ π μ ) cos( 3 2 3 ) cos( 2 3 ) ( ) sin( 2 3 ) 3 / 2 ( 3 ) / ( = = = + + N L L L O L L O t d t 3 2 2 = L L dc I ωl μ
Passive Mitigation Techniques DC Link Chokes (DC Bus Inductor) Makes discontinuous current continuous Small and inexpensive Does NOT Cause overlap phenomenon and so does not reduces dc bus voltage Does not help damp transient surges on line due to lightning and capacitor switching YASKAWA Page. 24
Passive Mitigation Techniques DC Link Choke (DC Bus Inductor) THD 80%. THD 37%. YASKAWA Page. 25
YASKAWA Page. 26 Waveform With DC Link Choke No overlap of Diode Conduction Ldc π π μ ω ω π π μ π μ N L L L O L L O t d t + + = = = 3 2 3 ) cos( 2 3 ) ( ) sin( 2 3 ) 3 / ( 2 3 ) / ( dc m m cr m m avg ph m cr I T i t L t i L 6 / 3 3 3 3 = Δ Δ = = = Δ Δ π π π π π
40.00 30.00 20.00 10.00-10.00-20.00-30.00 0 30.00 20.00 10.00-10.00-20.00-25.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 0 40.00 30.00 20.00 10.00 0-10.00-20.00-30.00 30.00 20.00 10.00-20.00-25.00 40.00 30.00 20.00 10.00 0-10.00-20.00-30.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 20.00 0-10.00 30.00 20.00 10.00-10.00-20.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 0 40.00 30. 00 20. 00 10. 00 0-10.00-20.00-30.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 30.00 10.00 0-10.00-20.00 25.00 20.00 15.00 10.00 5.00-5.00-10.00-15.00-20.00 0 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 25.00 20.00 15.00 10.00 5.00 0-5.00-10.00-15.00-20.00 40.00 30.00 20.00 10.00-10.00-20.00-30.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 0 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 30.00 20.00 10.00 0-10.00-20.00 20.00 10.00-10.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 0-20.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 25.00 20.00 15.00 10.00 5.00 0-5.00-10.00-15.00 20.00 15.00 10.00 5.00-5.00-10.00-15.00 0 20.00 15.00 10.00 5.00-5.00-10.00-15.00-20.00 0 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 25.00 20.00 15.00 10.00 5.00 0-5.00-10.00-15.00-20.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 20.00 10.00 0-10.00-20.00-25.00 4.98 4.99 4.99 4.99 4.99 4.99 5.00 5.00 5.00 AC Line Reactor vs DC Link Choke Input Current THD (%) Input Current THD (%) 120 100 80 60 40 20 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Inductance (mh) 120 THD=92.5% x Z=1% 100 Δ 80 60 40 20 0 THD=92.5% Lac=0.47mH THD=92.4% Z=1% THD=92.4% Ldc=0.47mH THD=44% Lac=0.47mH THD=44% Z=1% THD=59.6% Lac=1.4mH THD=50.6% Z=3% THD=37.3% Lac=1.4mH THD=82.9% Ldc=1.4mH 30.00 THD=37.3% Z=3% THD=33.8% Lac=2.35mH THD=59.6% Z=3% THD=58.5% Ldc=2.35mH THD=43.9% Lac=2.35mH DC reactor only (with Z SC =0.1%) AC reactor only AC reactor with DC reactor of 2.75mH 0 1 2 3 4 5 6 7 8 AC equivalent Z (% Impedance) YASKAWA Page. 27 THD=33.8% Z=5% x Δ THD=43.9% Z=5% 25.00 DC reactor only (with L IN =47μH) AC reactor only AC reactor with DC reactor of 2.75mH THD=37.7% Z=5%
AC Line Reactor vs DC Link Choke 640 620 x Δ DC reactor only (with L IN =47μH) AC reactor only Avg. DC Bus oltage () 600 580 560 540 520 500 0 1 2 3 4 5 Inductance (mh) YASKAWA Page. 28
Optimal Solution AC Line Inductor Alone Not Optimal because of oltage Drop DC Link Inductor Alone Does Not Provide Surge Protection Optimal Solution is a Combination of the two 1% AC Input Inductor + Standard DC Link Choke YASKAWA Page. 29
Harmonic Filters Capacitor Based Harmonic Filters Series Filter tuned to offer high impedance to select frequencies Shunt Filter tuned to shunt select frequencies Hybrid Filters combination of above Large, bulky, expensive, and often ineffective YASKAWA Page. 30
Series Harmonic Filter Series Filter Designed to handle rated load current More often found in single-phase applications to impede 3rd harmonic current L f C f YASKAWA Page. 31
Shunt Filter Shunt Harmonic Filter Designed to shunt select frequencies Draws fundamental frequency current L f resulting in leading A operation Need multiple section to be effective Does not distinguish between intended load and other loads C f YASKAWA Page. 32
Modified Shunt Harmonic Filter Shunt Filter with Series Impedance Add a series inductance to restrict import of harmonics MTE s and Mirus International s Filter Structure 5% AC Reactor 5% AC Reactor U W IM L f C f YASKAWA Page. 33
Hybrid (Broad Band) Harmonic Filter Combination of shunt and series filter Series inductance and Shunt Capacitor over-voltage problem Autotransformer used to solve this Bulky, expensive Capacitor switching needed Autotransformer L f C f YASKAWA Page. 34
Issues With Harmonic Filters All capacitor based shunt type filters draw leading current and cause over-voltage Power Loss, and Higher Stresses on DC Bus Capacitors avoid using this Generally multiple sections needed Bulky and Expensive Can cause system resonance YASKAWA Page. 35
Risk of Parallel Resonance Parallel Resonance X c X L i h i h Power Factor Capacitors Relieve Load Current measured at the capacitor, showing 660Hz, (11th harmonic resonance) Figure 36.2 Resonance occurs when: X c = X L YASKAWA Page. 36
Multi-pulse Harmonic Mitigation Technique 12-pulse Techniques Three-winding isolation transformer Hybrid 12-pulse Autotransformer based 12-pulse scheme YASKAWA Page. 37
Three Winding 12-pulse Scheme 3-winding isolation transformer X1 H1 H2 H3 H2 H1 H3 X3 Y2 Y1 X1 X2 X2 X3 Y1 Y2 Y3 Y3 L1 L2 L3 L11 L21 L31 L dc DC U W IM Rated for full power operation bulky but ONLY option when input is medium voltage and drive is of low voltage rating YASKAWA Page. 38
Three Winding 12-pulse Waveforms YASKAWA Page. 39
Hybrid 12-pulse Scheme I in I xfmr H1 H2 H3 Half power phase-shifting isolation transformer H1 X1 X2 X1 X2 X3 L1 L2 L3 L dc DC U W IM Optional input inductor, Lin H2 H3 I LM X3 L11 L21 L31 Matching Inductor (half-rated current) Transformer rated for half power attractive option YASKAWA Page. 40
Hybrid 12-pulse Waveforms I in I Lm I xfmr % Distortion 16 12 8 4 With L in With no L in THD= 6.7% with 5% input reactor; 8.8% with no input AC reactor. THD 5 7 11 13 17 19 Harmonic Order YASKAWA Page. 41
Autotransformer 12-pulse Scheme Inter-phase Transformer 3-ph ac supply b b zero-sequence blocking Transformer L1 L2 L3 Common Core L11 c A Cited as prior art in US patent# 4,255,784 C a L21 L31 Inter-phase Transformer c B a Autotransformer configuration Needs IPT and ZSBT bulky and costly YASKAWA Page. 42
Autotransformer 12-pulse Scheme 3-Ph AC supply R' S' T' FD #1 U W Motor 1 b' b A C c a' c' B a R S T FD #2 U W Motor 2 Autotransformer configuration If loads are isolated and fairly balanced, this is very attractive YASKAWA Page. 43
Four Winding 18-pulse Scheme 4-winding isolation transformer X1 L1 X1 X2 L2 H1 H2 H3 H1 H2 H3 X3 Y3 Y2-20 deg X2 Z1 Y1 X3 Y1 Y2 Y3 L3 L11 L21 L31 L dc DC U W IM Z3 +20 deg Z2 Z1 Z2 Z3 L12 L22 L32 Transformer is rated for full power operation - bulky and expensive Cost effective method if primary is M Attenuates conducted EMI effectively YASKAWA Page. 44
Autotransformer 18-pulse Scheme I in H1 H2 H3 18-pulse autotransformer H1 1 2 9 8 3 7 4 1 L1 2 L2 3 L3 4 5 6 I 5 I 1 I 3 L11 L21 L31 L dc DC U W IM Input inductor, Lin H3 6 5 H2 7 8 9 L12 L22 L32 Autotransformer configuration Patented by D. Paice only two US manufacturers licensed at present YASKAWA Page. 45
Autotransformer 18-pulse Scheme Needs three diode bridges Yaskawa uses external diode bridges makes it expensive Needs 7% input reactor for achieving THD levels of 5% and below increases cost and space With no input reactor, THD observed is about 8.8% YASKAWA Page. 46
Autotransformer 18-pulse Waveform I 1 I 3 I 5 I in % Distortion 16 12 8 4 With L in With no L in THD= 5.5% with 5% input reactor; 8.7% with no input AC reactor. THD 5 7 11 13 17 19 Harmonic Order YASKAWA Page. 47
Yaskawa s own 18-pulse Scheme Less Complicated Structure Low Cost because of standard configuration Needs only 1.5% input reactor to bring THD level to less than 5% With no input reactor, THD level observed to be 6.5% YASKAWA Page. 48
Hybrid 18-pulse Scheme by Yaskawa 2/3 rated phase-shifting isolation transformer Input inductor, Lin H1 H1 H2 H3 H2 H3 X3 Y2 X1-20 deg X2 Y1 +20 deg Y3 X1 L1 X2 L2 X3 L3 Y1 L11 Y2 L21 Y3 L31 L dc DC U W IM Power rating of isolation transformer is 2/3 of rated output power Current through matching inductor is 1/3 of rated input current Patent Pending Matching Inductor (1/3-rated current) YASKAWA Page. 49 L12 L22 L32
Hybrid 18-pulse Waveform LN I in % Distortion 16 12 8 4 With L in With no L in THD= 4.5% with 1.5% input reactor; 6.5% with no input AC reactor. THD 5 7 11 13 17 19 Harmonic Order YASKAWA Page. 50
Power Factor and Harmonics Two Definitions of Power Factor Exists Displacement Power Factor: Cosine of the angle between the fundamental voltage and fundamental current waveform For FDs, this value is almost always unity (0.99) YASKAWA Page. 51
Power Factor and Harmonics True Power Factor Ratio of True Power to Total olt-ampere kw Demanded by Load pf = ka Total olt-ampere includes A demanded by Harmonic Content in Waveform pf = pf = kw k I total dpf 1+ THD = k 2 kw I 2 1 + YASKAWA Page. 52 I 2 n = k I 1 kw 1+ THD 2
Power Factor and Harmonics True Power Factor is poor: THD 80% pf = dpf 1+ 0.8 2 = 0.78. THD 37% pf = 1+ dpf 0.37 2 = 0.937. YASKAWA Page. 53
Conclusions IEEE 519 does not mean THD < 5% Find out PCC, I SC, and I L, Apply the Spirit of IEEE 519 correctly YASKAWA Page. 54
Conclusions Avoid capacitor based harmonic filter 12-pulse techniques can achieve low TDD at drive input Hybrid 12-pulse is attractive, less bulky and cost-effective Isolation transformer based method is best when input is M Use 18-pulse only when Customer demands (Less than 5% TDD) Hybrid 18-pulse is attractive Isolation transformer based method is best when input is M YASKAWA Page. 55