Power Quality Considerations When Applying Adjustable Frequency Drives Explanations and Various Countermeasures: 7/15/2002 PP.AFD.08 1 of 28 Power Quality Considerations When Applying Adjustable Frequency Drives Explanations and Various Countermeasures
Power Quality: 7/15/2002 PP.AFD.08 2 of 28 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
Power Quality Topics: 7/15/2002 PP.AFD.08 3 of 28 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!
Definition of Harmonics: 7/15/2002 PP.AFD.08 4 of 28 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.
What is Harmonic Distortion?: 7/15/2002 PP.AFD.08 5 of 28 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 Every Wave shape has Harmonic Distortion! THD = 1.2% THD = 78.3%
PowerPoint Presentation: 7/15/2002 PP.AFD.08 6 of 28 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 6-1. Figure 6 .1 Normalized Harmonic Spectrum Figure 6 .2
Types of Harmonics: 7/15/2002 PP.AFD.08 7 of 28 Types of Harmonics DC Drive - SCR Based AC Drive - Diode Rectifier New Technology May Solve Old Power Quality Problems SCR Rectification - Line Notching, Increases Voltage Distortion Diode Rectification - Pulsed Current, Increases Current Distortion
Possible Effects of Harmonics: 7/15/2002 PP.AFD.08 8 of 28 Possible Effects of Harmonics Increased Transformer Heating 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 Harmonic Distortion most likely will have no effect on Power Distribution Performance
PowerPoint Presentation: 7/15/2002 PP.AFD.08 9 of 28 C onsider 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. How Harmonics Lower Efficiency Figure 9 .1 pf = kW/kVA I THD = 92.8% pf = 1/Sqrt(1 2 +.928 2 ) pf = 73.3% Power Factor Considering 92.8% I THD Figure 9 .2 True factor is improved, when current distortion is limited by system impedance. (Including reactors, or bus chokes.) Figure 9 .1 pf = kW/kVA I THD = 32.6% pf = 1/Sqrt(1 2 +.326 2 ) pf = 95.08! Power Factor Considering 32.6% I THD Figure 9 .2 Impedance Improves Efficiency!
PowerPoint Presentation: 7/15/2002 PP.AFD.08 10 of 28 Power Factor When Harmonics Exist From IEEE Std. 141-1993: Power is the product of in-phase current times the voltage or: P 60 = V 60 * I 60 cos q In the case of harmonics: P h = V h * I h cos q or S = (Sqrt(P 2 + Q 2 +D 2 )) {R1] Where P = Real Power, Q = Reactive Power and D = Distortion Power. System losses will be higher due to the harmonic components, than with equivalent 60 kVA. P h = I 2 h * R h (Ohm’s law in harmonic-land) True Power Factor Representation - Expanded Apparent Power (kVA)* P Real Power (kW) Q Reactive Power X (kVAr) D Distortion Figure 10 .1
PowerPoint Presentation: 7/15/2002 PP.AFD.08 11 of 28 The Risk of Parallel Resonance H p - the harmonic order (per-unit frequency) at parallel resonant frequency MVA sc - the system short-circuit capacity MVAr c - the power factor improvement capacitor H p - Sqrt(MVA sc / MVAr c ) Per IEEE Red Book (Std. 141-1993): “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. X c X L i h i h Resonance occurs when: X c = X L Power Factor Capacitors Relieve Load [R2] Figure 11 .1 Parallel Resonance Current measured at the capacitor, showing 660Hz, (11th harmonic resonance) Figure 11 .2
What Guidance is there in Industry?: 7/15/2002 PP.AFD.08 12 of 28 What Guidance is there in Industry? IEEE 519 - 81 UTILITY USER A USER B VTHD A VTHD B
IEEE519-92: 7/15/2002 PP.AFD.08 13 of 28 IEEE519-92 Specification has created a problem Compare Short Circuit Capacity to Maximum Load Current Determine Point of Common Coupling
IEEE519-92: 7/15/2002 PP.AFD.08 14 of 28 IEEE519-92 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 Don’t Make Your Customer Pay for Poor Guidance 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! Issues:
Countermeasures with Yaskawa Drives: 7/15/2002 PP.AFD.08 15 of 28 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
PowerPoint Presentation: 7/15/2002 PP.AFD.08 16 of 28 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
PowerPoint Presentation: 7/15/2002 PP.AFD.08 17 of 28 DC Link Choke Reduce Distortion by 50% from standard 6 Pulse Drive Represent 3% input impedance to Line Power Current Waveform
PowerPoint Presentation: 7/15/2002 PP.AFD.08 18 of 28 AC Input Reactor Reduce Distortion by 50%, dependent on System Impedance Reduce Nuisance Trips from surge voltages Current waveform
PowerPoint Presentation: 7/15/2002 PP.AFD.08 19 of 28 Low Pass / Broad Band Filters Designed to take 5th and 7th Harmonic out of system Series Resonant Tuned Leading Power Factor
PowerPoint Presentation: 7/15/2002 PP.AFD.08 20 of 28 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. 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. Typical de-tuned p.f. bank Figure 20 .1 Drive Applied Harmonic Filter Figure 20 .2
PowerPoint Presentation: 7/15/2002 PP.AFD.08 21 of 28 12-Pulse Transformer Reduce Distortion by 92% Lowest Levels in the Industry Input Current Waveform
PowerPoint Presentation: 7/15/2002 PP.AFD.08 22 of 28 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 To DC Bus From DC Bus From Source Transformer Rectifiers Figure 22 .2
Example!: 7/15/2002 PP.AFD.08 23 of 28 Example! Information Needed: - Simplified One Line Diagram - Electrical Schedule (preferred) - Ratio of linear load / Non Linear Load Quick Estimate based upon Electrical Schedule System Properties: Connected kVA = 769 Impedance: 5.75% Non Linear kVA = 114 Xfmr Size = 1000 kVA
PCC Service at Entrance of Bldg.: 7/15/2002 PP.AFD.08 24 of 28 PCC Service at Entrance of Bldg. Power System Distribution Specification 1000 Kva @ 5.75 % impedance PCC = Service Entrance 655 Hp Linear Load ISC/IL = 22.7 115 Hp MagneTek Drive Load Power Distribution Rule of Thumb: Keep Voltage Distortion Below 5% - Normal Conditions 7.5% Start Up, Unusual Conditions IEEE519-92, ITHD < 8.0%
PCC at VFD Terminals: 7/15/2002 PP.AFD.08 25 of 28 PCC at VFD Terminals Power System Distribution Specification 1000 Kva @ 5.75 % impedance PCC = At VFD Terminals 115 Hp MagneTek Drive Load ISC/IL = 153 Power Distribution Rule of Thumb: Keep Voltage Distortion Below 5% - Normal Conditions 7.5% Start Up, Unusual Conditions IEEE519-92, ITHD < 15.0%
Summary: Effects of Harmonics: 7/15/2002 PP.AFD.08 26 of 28 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 = 3111 - 3021 = 89 kVA = 2.8 % Losses at Full Capacity Simple Calculations to put End User at Ease!
PowerPoint Presentation: 7/15/2002 PP.AFD.08 27 of 28 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.
Benefits Outweigh Challenges: 7/15/2002 PP.AFD.08 28 of 28 Benefits Outweigh Challenges 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 Educating on Power Quality Electrical Noise Issues Motor Design Compatibility Speed Windings Long Lead Length Retrofits Benefits Challenges