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Power Quality and Harmonics: Causes, Effects and Remediation Techniques Carol Gowan Chad Loomis, PE

2. Overview. What is Power Quality?What are some power quality concerns?What are Harmonics and what causes them?What effects do Poor PQ and Harmonics have on a building power system and components?How can Power Quality and Harmonics Issues be mitigated?Questions. 3. What is Power Quality?. Can

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Power Quality and Harmonics: Causes, Effects and Remediation Techniques Carol Gowan Chad Loomis, PE

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    1. 1 Power Quality and Harmonics: Causes, Effects and Remediation Techniques Carol Gowan Chad Loomis, PE Cornell University PDC 12/13/2006 Electrical Design Section

    2. 2 Overview What is Power Quality? What are some power quality concerns? What are Harmonics and what causes them? What effects do Poor PQ and Harmonics have on a building power system and components? How can Power Quality and Harmonics Issues be mitigated? Questions

    3. 3 What is Power Quality? Can be defined as: “Any power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment”

    4. 4 What are general classes of power quality problems per IEC? Conducted low-frequency phenomena Signal systems (power line carrier) Voltage fluctuations (flicker) Voltage dips and interruptions Voltage Imbalance (unbalance) Power frequency variations Induced low-frequency voltages DC in ac networks Harmonics, interharmonics Radiated low-frequency phenomena Magnetic and Electric Fields

    5. 5 Categories and Characteristics of Power System Electromagnetic Phenomena (IEC)

    6. 6 Voltage Fluctuations within Cornell’s Utility Distribution Transients – very quick < 1 cycle Normal cause is lightning strike No lights flicker Cornell Utilities employs lightning arrestors at substations and at primary switches located at each building electric service equipment End users need to purchase/install TVSS equipment to further clamp the voltage spike.

    7. 7 Sags / Swells Voltage imbalance lasting from 3-20 cycles Typical cause ? NYSEG switching on the incoming 115kV transmission line Lights flickering are indicative of this fault Utilities does not protect for this condition

    8. 8 Long term voltage fluctuations As load increases, voltage drops (and vice versa) Cornell Utilities compensates the long-duration voltage variations through the use of automatic load tap changers at the Maple Ave. substation System voltage tolerance limits are set in ANSI C84.1. The Cornell system voltages are designed to always operate in the range ‘A’ limits (108 – 126V) * refer to IEEE 141-1993

    9. 9 Harmonic Distortion Typically generated within a facility, not a distribution issue Utilities does not protect for this condition

    10. 10 CBEMA curve for susceptibility of 120V Computer Equipment Computer Business Equipment Manufacturers Association Early 1980’s CBEMA designed the curve to point out ways in which system reliability could be provided for electronic equipment

    11. 11 ITIC curve for susceptibility of 120V Computer Equipment Information Technology Industry Council This derivation was developed to show a curve that more accurately reflects the performance of typical single-phase, 60-Hz computers and their peripherals

    12. 12 Computer Equipment Disturbance Table (Dranetz-BMI Field handbook for PQ Analysis)

    13. 13 What are NOT proper descriptors of power quality problems per IEC? Blackout Blink Brownout Bump Clean Ground Clean Power Dirty Ground Dirty Power

    14. 14 What are the causes of most trouble tickets issued for computer problems at Cornell? Internally (building area specific) induced problems Shared neutrals Overloaded circuits (breakers tripping) Poorly or improperly grounded circuits Note there are VERY minimal utility / delivery issues (we are very fortunate here at Cornell!) Handful of harmonic issues in the last 20 yrs, but harmonics are becoming a larger concern with more sophisticated systems and buildings.

    15. 15 What Are Harmonics? “A component frequency of a harmonic motion of an electromagnetic wave that is an integral multiple of the fundamental frequency” US fundamental frequency is 60 Hertz 3rd Harmonic is 3 x 60Hz or 180Hz 5th Harmonic is 5 x 60Hz or 300Hz, etc.

    16. 16 What Causes Harmonics? Non-Linear Loads Current is not proportional to the applied voltage

    17. 17 Linear loads and current waveforms. Pure resistance, inductance, and capacitance are all linear. What that means: If a sine wave voltage of a certain magnitude is placed across a circuit containing pure resistance, the current in the circuit follows Ohm's Law: I = E ÷ R. So, for a specific value of ohms, the relationship of volts and amperes is a straight line. The current will always be a sine wave of the same frequency. Linear Loads include Incandescent lighting, heating loads, and motors Linear vs. Non-linear loads and current waveforms

    18. 18 Linear vs. Non-linear loads and current waveforms Nonlinear loads and current waveforms. Solid state electronics is based on the use of semiconductors. These materials are totally different in that their response to voltage is not a straight line. What this means: With a nonlinear load, you cannot easily predict the relationship between voltage and current — unless you have an exact curve for each device. With equipment containing many solid-state devices, such an approach is impossible. Nonlinear loads are switched on for only part of the cycle, as in a thyristor-controlled circuit, or pulsed, as in a controlled-rectifier circuit.

    19. 19 Effect of harmonics on waveform

    20. 20 What do harmonics do? Harmonics are carried through the system from the source and can nearly double the amount of current on the neutral conductor in three phase four wire distribution systems. Distorted currents from harmonic-producing loads also distort the voltage as they pass through the system impedence. Therefore, a distorted voltage can be presented to other end users on the system. Overall electrical system and power quality is affected by the introduction of harmonics.

    21. 21 Sources of Harmonics Solid State Electronic Devices which contain a poor power supply Computers (PCs/CPUs) Laser Printers Copy Machines *Solid State UPS Units Solid State Devices (Fluorescent lighting ballasts) **Rectifiers (AC-DC Converters ? VFDs) Welding Units Arc Furnaces

    22. 22 What are the order of typical harmonics generated by non-linear loads?

    23. 23 Effects of Harmonics Distorted Voltage Overheated Transformers and Motors Increases Hysteresis (magnetization) losses in steel and iron cores of transformers, motor and magnetic trip units of circuit breakers (Equipment inefficiencies and overheating) Heating of Neutral Conductors Skin Effect ? Increased amount of current flowing on the outside of conductors (overheating) Low Voltage at End Loads High Neutral to Ground Voltages at End Loads

    24. 24 Effects of Harmonics (cont) Operation Problems of Relays and Circuit Breakers Thermal/Magnetic Trip Circuit Breakers Fuses & bimetal strips respond to True RMS Harmonic currents increase eddy current losses in the core steel of the metallic strip. This causes an OVERprotection situation… Increased losses generate additional heat, this effect the thermal trip of the unit. Electronic Trip Circuit Breakers Magnitude and phase angle(s) of harmonic current(s) in relationship to the fundamental current can cause: Overprotection when: Peak current sensing > True RMS Underprotection when: Peak current sensing < True RMS Changing power system loads will vary the magnitude and phase angle, resulting in inaccurate and unpredictable sensing units and overload protection

    25. 25 Effects of Harmonics (cont) Communication Problems If sharing common parallel path, potential for harmonics to have inductive coupling effect on unshielded cabling Current Measurement Problems (distorted waveform) Unreliable Operation of Electronic Equipment Mis-operation of electronic equipment that measures frequency or uses the zero crossing point of a sine wave. Control of Speed and Voltage Problems on Emergency Generators (supplying power) Capacitor Bank Application Problems (heating) Computer (PC/CPU) data errors / data loss Affects power supplies and sensitive electronics

    26. 26 How can Harmonics be Reduced? Isolate harmonic loads on separate circuits (with or without harmonic filters) Harmonic mitigating transformers Phase shifting (zig-zag) transformers Used to cancel out specific harmonics by making one voltage circuit 180 degrees out-of-phase Filter capacitor backs

    27. 27 How can Harmonics be Reduced? For VFD and UPS specifically: Line Reactors K-Rated / Drive Isolation Transformers Harmonic Mitigating / Phase Shifting Transformers 12, 18 or 24 pulse Converters Passive parallel / series tuned Filters Active Filters

    28. 28 How can Harmonics be Reduced? PROPER GROUNDING Neutral to ground conductor connection at one location; at main panel or transformer secondary When neutral is connected to ground at multiple locations, interference can occur with sensitive electronic devices. Run power and control conductors in separate raceways Sensitive loads should not share neutral and ground conductors. Avoid using conduit as the ground return path, run dedicated ground wire with circuit conductors Refer to IEEE Std 1100-1992 (Emerald Book)

    29. 29 Computer Equipment Grounding TOP ? Radial or “daisy chain” grounding Although Code compliant per NEC, small differences in potential can cause unintended ground loops BOTTOM ? Best method for grounding is via dedicated equipment grounding conductors back to the source

    30. 30 IEEE Defined Harmonic Current Limits

    31. 31 Voltage Limit / HarmonicsEvaluation Procedure

    32. 32 Questions? Thank You! Cornell Planning, Design & Construction Carol Gowan 254-1457 cag57@cornell.edu Chad Loomis 255-8039 cel36@cornell.edu

    33. 33 Simplified Distribution System Diagram

    34. 34 Simplified Distribution System Diagram

    35. 35 Simplified Distribution System Diagram

    36. 36 Simplified Distribution System Diagram

    37. 37 Simplified Distribution System Diagram

    38. 38 Simplified Distribution System Diagram

    39. 39 Simplified Distribution System Diagram

    40. 40 Simplified Distribution System Diagram

    41. 41 Simplified Distribution System Diagram

    42. 42 Simplified Distribution System Diagram

    43. 43 Simplified Distribution System Diagram

    44. 44 Typical PQ Evaluation Process

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