6 Pulse SCR Drives. Discussion Highlights. Six Pulse SCR Rectifier with DC Link Inductor. Notch filter for 6 pulse SCR. Harmonic Study Example

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1 Discussion Highlights Harmonics in Electrical Power ystems: Effects of New Technologies Capitol Mechanical Engineering & Energy ymposium Thursday October 17th, Peter E. utherland, Ph.D., P.E. ix-pulse Rectifiers Reactor Current ource (CR Drives) Capacitor Voltage ource ( Drives) Active Front End Voltage ource ( Drives) Eighteen Pulse Rectifiers Multiple Transformers ingle Transformers Harmonic Filters Passive Filters Active Filters 2 ix Pulse CR Rectifier with Inductor 6 Pulse CR Drives AC Line Current Most references still consider the standard six-pulse CR rectifier as the typical harmonic load. Harmonic magnitudes 1/h Current THD 25% Displacement Power Factor 80% This may lead to application errors by severely underestimating or overestimating harmonic current magnitudes and power factor. 3 Notch filter for 6 pulse CR Harmonic tudy Example Filter R F +j(x L -X C ) Reduces harmonics for most 6-pulse drives Provides power factor compensation Tuned for h= Impedance (Ohms) Harmonic Water Treatment Plant 2 x 300 HP Motors VFD s w/line & Load Reactors Harmonic Filter to be specified 6 1

2 Power Factor Calculations Harmonic Filter Calculations ource Capacitor increases power factor from 80% 97% Capacitor causes Resonance near 5 th harmonic Voltage on 480 V Bus is unchanged Filter shifts resonance from 5 th 8 th harmonic Filter increases power factor from 80% 95% Voltage on 480 V Bus is unchanged. ource Filter C VFD Impedance (Ohms) VFD Impedance (Ohms) Harmonic Harmonic w/c w/o C Filter w/c w/filter urprise After Installation of Notch Filter Learning Assessment 1 Power factor goes Leading Voltage on 480 Volt Bus rises by 10% Actual drive was 6-Pulse type with 98% Power Factor! Notch Filter had to be Removed! tudy had to be re-done with Harmonics and no power factor correction Possible solutions: Use Broadband or Active Filter (to be discussed later) The fifth harmonic of a six-pulse CR rectifier has a magnitude of what % of the fundamental? 9 10 Drive: Diode Front End Drives with Diode Front End ource Impedance X v AC Line Reactor Diode Rectifier Current THD % Displacement Power Factor 98% x c 480V Bus Capacitor Inverter and Motor Modeled as current source 11 2

3 Drive: Modes of Operation Drive: Reactors decrease AC line harmonics h=5 90% Discontinuous Current Mode (DCM) Higher harmonics Continuous Current Mode (CCM) Lower harmonics i h /% DCM CCM h=5 30% x v /% Drive: Power Factor increases with reactance Learning Assessment 2 cos 1 = Displacement Power Factor (cosine of angle between fundamental voltage and current) cos 1 98% = kw/kva = Total Power Factor cos 1 when harmonics are present The higher the AC line reactance: the lower the displacement power factor, the higher the total power factor. The fifth harmonic of a Drive with Diode Front End can have a magnitude as much as what % of the fundamental? 16 Drive: Active Front End Drives with Active Front End X ource Impedance X ac Line Reactor X dc = ~ Reactor Capacitance AC current is sinusoidal with high frequency harmonics Current THD < 5% Voltage THD < 3% Displacement Power Factor = 100% Meets IEEE without harmonic filters. 17 3

4 Active Front End Line Current Harmonics Learning Assessment 3 A Drive with Active Front End has current harmonics less than what %? Pulse Drive for Harmonic Reduction Drives with 18-Pulse Rectifiers quare wave harmonic current spectrum approximately 1/h magnitude. n=18 pulses per cycle h = np 1, n=1, 2, 3, = 17, 19, 35, 37,. Current THD < 5%, Voltage THD < 3 % Displacement Power Factor 92-99% Pulse Drive with 3 transformers 18-Pulse Drive with Autotransformer Three-phase to ninephase transformation. Autotransformer is significantly less expensive than threetransformer design. imilar harmonic spectrum as conventional 18-pulse drive. 4

5 Example: Power Plant Cooling Water Pumps Difficult pecifications Comply with phase transformer inside 18 pulse drive 3% VTHD 5% ITHD 18 PULE NO FILTER Two 700HP 440V Cooling Water Pumps in a Power Plant. PCC I DRIVE BU CONNECTION Harmonic Measurements Line Reactor olution 18-Pulse Rectifier Current Harmonics 0.40 Harmonic Current pu of Fundamental VFD Current Waveform ITHD=37.6% Harmonic Order - H Current harmonics greatly exceeded those expected of an 18-pulse drive, due to Voltage Unbalance Voltage unbalance: V A =258.6, V B =256.5, V C =257.9 %V unbalance = (V max -V min )/Avg = 2.1/257.7 =0.8% Adding 5% Line Reactors in EMTP simulation shows ITHD drops to 7.6% Line Reactors Added Learning Assessment 4 The lowest harmonic of an 18-pulse drive is what multiple of the fundamental? Line Reactors did not fit in drive location. Had to be mounted on wall near ceiling in adjacent room 30 5

6 ummary of Drives Rectifier Inverter AC Line Current CR Passive Diode Passive Diode Passive IGBT Active 18 Pulse Diode CI CR VI VI VI VI quare Wave Discontinuous Continuous ine Wave ine Wave AC Line VTHD AC Line ITHD Power Factor Reactor 5% 25% 80% Capacitor 37% 97% 95% Capacitor 11% 25% 98% Capacitor <3% <5% 100% Capacitor <5% <3% 92-99% Broadband Filters 32 Broad-Band Filter Broadband filter gives lower harmonics than 18-pulse drive Filter connected between bus and drive. Used with diode or CR inputs mall capacitor bank good for unity PF drives Reduces THD to <6% for drive More effective than 18-Pulse Drive, Choke or AC Line Reactor. Active Filters for Low Voltage Active Filters Filter connected between bus and drive. Not on bus like notch filters Used with diode or CR inputs mall capacitor bank good for unity PF drives Reduces THD to <6% for drive Provides full Compliance with IEEE

7 Active Filters for Low Voltage Learning Assessment 5 What is the difference between an active filter and a passive filter? 38 ummary ix-pulse Rectifiers Reactor Current ource (CR Drives) Normal 1/h harmonics, notch filter usually sufficient Capacitor Voltage ource ( Drives) High magnitude harmonics, unity power factor Active Front End Voltage ource ( Drives) Harmonics within IEEE Eighteen Pulse Rectifiers Harmonics increased by voltage unbalance Harmonic Filters Passive filters: notch or broadband Active filters cancel out harmonics Thank you! Any questions?