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1 Power Quality Considerations When Applying Adjustable Frequency Drives Explanations and Various Countermeasures 7/15/2002 PP.AFD.08 1 of 28

2 Power Quality Why the Renewed Interest in Power Quality? Copy Machines Fax machines Computers Elevator Controls Solid State Lighting Ballasts Devices that incorporate Static Power Converters - SCRs, Diodes.. etc. Adjustable Frequency Drives Common Issue among Common Devices 7/15/2002 PP.AFD.08 2 of 28

3 Power Quality Topics What are Harmonics? What is Harmonic Distortion? Differences between current and voltage distortion Possible effects of Harmonics What Guidance is there in the Industry What Solutions does Yaskawa Offer? Harmonics are important to understand the relationship between Power Quality and switch mode power supplies! 7/15/2002 PP.AFD.08 3 of 28

4 Definition of Harmonics Harmonics are defined as currents or voltages with frequencies that are integer multiples of the fundamental power frequency SIMPLY PUT - Harmonics are used to mathematically describe the shape of a curve that is not sinusoidal. 7/15/2002 PP.AFD.08 4 of 28

5 What is Harmonic Distortion? Harmonic Distortion is a mathematical way of describing how non-sinusoidal a wave shape appears Fourier Analysis - Sum of the Squares TVD = V 2 h h= z THD = 78.3% THD = 1.2% Every Wave shape has Harmonic Distortion! 7/15/2002 PP.AFD.08 5 of 28

6 Fourier Analysis Fourier Analysis of the waveforms found in a three phase diode rectifier shows low order harmonics including the 5th, 7th, 11th, 13th, etc. Calculation of true power factor considers the energies contained on these additional frequencies. Figure 6-2 shows the resulting harmonic spectrum based on Fourier analysis of the current waveform shown in figure Magnitu de (as % of Fundamental Figure % Three phase diode rectifier, line voltage/current 30.38% Voltage Current Normalized Harmonic Spectrum 5.55% 7.16% 4.83% 4.32% 3.59% Harmonic Order Figure 6.2 7/15/2002 PP.AFD.08 6 of 28

7 Types of Harmonics DC Drive - SCR Based AC Drive - Diode Rectifier SCR Rectification - Line Notching, Increases Voltage Distortion Diode Rectification - Pulsed Current, Increases Current Distortion New Technology May Solve Old Power Quality Problems 7/15/2002 PP.AFD.08 7 of 28

8 Increased Transformer Heating Possible Effects of Harmonics recommended K-Factor of 4 to 13 on new installations Increased Conductor Heating larger gauge wire run two wires in parallel Electromagnetic Equipment PLCs - more sensitive to Voltage Notching System resonance - Power Factor Correction utilize input reactors to reduce likelihood of resonance Lower Power Factor for System ( ) PF = PF = Power / Power + Power + Power True Total Real Real Re act. Harmonics Harmonic Distortion most likely will have no effect on Power Distribution Performance 7/15/2002 PP.AFD.08 8 of 28

9 How Harmonics Lower Efficiency Consider estimating power factor at the terminals of an AC Drive in a system with low source impedance (high available short circuit current) with no input line reactor or DC bus choke. 500 True factor is improved, when current distortion is limited by system impedance. (Including reactors, or bus chokes.) Figure Figure 9.1 Power Factor Considering 92.8% I THD pf = kw/kva I THD = 92.8% pf = 1/Sqrt( ) pf = 73.3% Figure 9.2 Power Factor Considering 32.6% I THD pf = kw/kva I THD = 32.6% pf = 1/Sqrt( ) pf = 95.08! Figure 9.2 Impedance Improves Efficiency! 7/15/2002 PP.AFD.08 9 of 28

10 Power Factor When Harmonics Exist From IEEE Std : Power is the product of in-phase current times the voltage or: P 60 = V 60 * I 60 cos θ In the case of harmonics: P h = V h * I h cos θ or S = (Sqrt(P 2 + Q 2 +D 2 )) {R1] Where P = Real Power, Q = Reactive Power and D = Distortion Power. True Power Factor Representation - Expanded Q Reactive Power X (kvar) System losses will be higher due to the harmonic components, than with equivalent 60 kva. P h = I 2 h * R h P Real Power (kw) Figure 10.1 (Ohm s law in harmonic-land) 7/15/2002 PP.AFD of 28

11 The Risk of Parallel Resonance Hp - the harmonic order (per-unit frequency) at parallel resonant frequency MVAsc - the system short-circuit capacity MVArc - the power factor improvement capacitor Power Factor Capacitors Relieve Load [R2] X XL Hp - Sqrt(MVAsc / MVArc) c ih ih Per IEEE Red Book (Std ): If the SCR (short circuit ratio is less than 20), and there is a parallel resonance condition near a characteristic harmonic of the non-linear load, there will be a problem. Since all power systems have inductance and capacitance, they will resonate at a given frequency. When an exciting energy at that frequency, in a quantity that is large enough to offset the natural dampening in the system, is present, resonance will occur. In a typical industrial or commercial building, the natural resonance frequency is likely to be in the range of 1000 Hz. It is a real world possibility that a system can resonate. F igure 11.1 Resonance occurs when: Xc = XL Parallel Resonance Current measured at the capacitor, showing 660Hz, (11th harmonic resonance)f igure /15/2002 PP.AFD of 28

12 Table Two: Voltage Distortion Limits Bus Voltage at PCC Individual Voltage Distortion (%) What Guidance is there in Industry? Total Voltage Distortion THD (%) 69 kv and below kv through 161 kv kv and above UTILITY VTHD A VTHD B USER A USER B IEEE /15/2002 PP.AFD of 28

13 IEEE Table Three: Current Distortion Limits for General Distribution Systems (120 V through 69 kv) Maximum Harmonic Current D istortion in Percent of Load Current I SC /I L <11 11 h<17 17 h<23 23 h<35 35 h TDD < < < < > Even harmonics are limited to 25% of the odd harmonic limits above. where ISC = Maximum short-circuit current at PCC. IL = Maximum demand load current (fundamental frequency component) at PCC. Compare Short Circuit Capacity to Maximum Load Current Determine Point of Common Coupling Specification has created a problem 7/15/2002 PP.AFD of 28

14 IEEE Issues: Addressed Current Distortion Does not clarify PCC Limits do not increase sufficiently closer to non-linear device Does not clarify injected harmonics Is not clear that goal is reasonable Voltage Distortion Results: Pass / Fail Dependant on Point of Measurement Specification is not intended to comply internally All electrical products are lumped together Double the price of a VFD package to make a waveform look Pretty! Don t Make Your Customer Pay for Poor Guidance 7/15/2002 PP.AFD of 28

15 Countermeasures with Yaskawa Drives Correctly Sized Input Transformer (Kfactor Rated) Standard AFD DC Link Choke (Standard from 30 Hp to 250 Hp) AC Input Reactor (Optional on All Drives) Custom Designed Low Pass or Broad Band Filters (Optional on All Drives) 12-Pulse Transformer (Optional from 30 Hp thru 1000 Hp The Solution to fit the Specification 7/15/2002 PP.AFD of 28

16 Standard 6-Pulse Front End Distortion Levels can vary from 60% to 130% Dependent on stiffness of power transformer Not a problem Input Current Waveform C u r re n t A m p s /15/2002 PP.AFD of 28

17 DC Link Choke Reduce Distortion by 50% from standard 6 Pulse Drive Represent 3% input impedance to Line Power Current Waveform C u r r e n t A m p s /15/2002 PP.AFD of 28

18 AC Input Reactor Reduce Distortion by 50%, dependent on System Impedance Reduce Nuisance Trips from surge voltages Current waveform C u r r e n t A m p s /15/2002 PP.AFD of 28

19 Low Pass / Broad Band Filters Designed to take 5th and 7th Harmonic out of system Series Resonant Tuned Leading Power Factor f = 1 ( 2π LC) 7/15/2002 PP.AFD of 28

20 Shunt Filters The development of shunt filters to correct harmonic distortion has lead to the acceptance of two common technologies, under one heading, but with dramatic differences. 1.) Power factor capacitor providers often use tuning reactors to de-tune power factor back, and apply it on a bus. These filters require careful field evaluation and harmonic analysis to assure effectiveness and prevent against resonance. Some are tuned to 240Hz, and are simply called de-tuned p.f. banks, others are tuned to 300Hz, and act as a shunt filters for harmonic energies. Ty pical de-tuned p.f. bank Figure 20.1 Drive Applied Harmonic Filter 2.) Drives applied filters which include a 5% series inductor and are applied in front of a single drive load, and correct harmonic distortion for that particular drive. Drive applied filters generally do not require rigorous system analysis, and there is not a risk of resonance. Figure /15/2002 PP.AFD of 28

21 12-Pulse Transformer Reduce Distortion by 92% Lowest Levels in the Industry Input Current Waveform C u r r e n t A m p s /15/2002 PP.AFD of 28

22 Multi-pulse Converters Using Phase Shifting Transformers Theoretical phase cancellation in transformer primary eliminates low harmonics. In practice, phase cancellation is dependant on current and voltage phase balance and current sharing between bridges. Can limit the level of current harmonic distortion to 5-15% depending on transformer configuration, bridge symmetry, and source impedance.. Twelve-pulse Drive using Zig-Zag Transformer From Source Figure 22.2 Transformer To DC Bus From DC Bus Rectifiers 7/15/2002 PP.AFD of 28

23 Example! Information Needed: - Simplified One Line Diagram - Electrical Schedule (preferred) - Ratio of linear load / Non Linear Load System Properties: Connected kva = 769 Impedance: 5.75% Non Linear kva = 114 Xfmr Size = 1000 kva Quick Estimate based upon Electrical Schedule 7/15/2002 PP.AFD of 28

24 Power System Distribution Specification % impedance PCC = Service Entrance 655 Hp Linear Load ISC/IL = Hp MagneTek Drive Load Harmonic Reduction PCC Service at Entrance of Bldg. 15% 10% 13% IEEE519-92, ITHD < 8.0% 5% 0% 4% 5% 5% 2% 1% 2% 1% 1% 1% 6 Pulse DC Link AC Input Reactor Low Pass Filter 12 Pulse Xfmr Power Distribution Rule of Thumb: Keep Voltage Distortion Below 5% - Normal Conditions 7.5% Start Up, Unusual Conditions 7/15/2002 PP.AFD of 28

25 Power System Distribution Specification % impedance PCC = At VFD Terminals 115 Hp MagneTek Drive Load ISC/IL = % 80% 70% 60% 50% 40% 30% 20% 10% 0% 87% Harmonic Reduction 34% 33% PCC at VFD Terminals 13% 8% 4% 2% 1% 1% 1% 6 Pulse DC Link AC Input Reactor IEEE519-92, ITHD < 15.0% Low Pass Filter 12 Pulse Xfmr Power Distribution Rule of Thumb: Keep Voltage Distortion Below 5% - Normal Conditions 7.5% Start Up, Unusual Conditions 7/15/2002 PP.AFD of 28

26 Summary: Effects of Harmonics #1 Effect of Harmonic is increased heating of supply transformer System Properties: Xfrm Rating: 2500 / 3125 FC kva Connected HP: 2785 Impedance: 5.67% Demand Hp: 1950 Estimated ITHD = 23.9% I Load = 3639 Amps Estimated Current Harmonics : I Load * THD = 870 Amps IRMS = (I Load^2 + I Harm ^2)^.5 = 3742 Amps Supply Xfmr KVA = Voltage * Current * 1.73 = 480 * 3742 * 1.73 = 3111 kva Capacity Reduction due to Current Harmonics KVA = Voltage * Current * 1.73 = 480 * 3639 * 1.73 = 3021 kva Difference = = 89 kva = 2.8 % Losses at Full Capacity Simple Calculations to put End User at Ease! 7/15/2002 PP.AFD of 28

27 Comparative Cost of Harmonic Mitigation Devices Figure 27.2 Assumptions: 10,000 installed base cost of 6-pulse drive. Values will vary for lower HP drives. 7/15/2002 PP.AFD of 28

28 Benefits Outweigh Challenges Educating on Power Quality Electrical Noise Issues Motor Design Compatibility Speed Windings Long Lead Length Retrofits Improve Building Owner s Bottom Line Thru Energy Efficiency Reduce Wear /Tear on Pumps, Belts, Seals, Bearings etc Eliminate Demand charge due to Inrush from Line starting Motor Gain Precise Control on Varying Climate in Building thru Automation 7/15/2002 PP.AFD of 28