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

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Power Quality and HarmonicsCauses, Effects and
Remediation Techniques Carol
Gowan Chad Loomis, PE Cornell
University PDC 12/13/2006 Electrical Design
Section
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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

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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

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What are general classes of power quality
problems per IEC?
(Intl Electrotechnical Commission)

• 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

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Categories and Characteristics of Power System
Electromagnetic Phenomena (IEC)
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Voltage Fluctuations withinCornells Utility
Distribution

• Transients very quick
• 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.

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Voltage Fluctuations withinCornells Utility
Distribution

• 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

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Voltage Fluctuations withinCornells Utility
Distribution

• 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

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Voltage Fluctuations withinCornells Utility
Distribution

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

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CBEMA curve for susceptibility of 120V Computer
Equipment

• Computer Business Equipment Manufacturers
  Association
• Early 1980s CBEMA designed the curve to point
  out ways in which system reliability could be
  provided for electronic equipment

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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

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Computer Equipment Disturbance Table(Dranetz
-BMI Field handbook for PQ Analysis)
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What are NOT proper descriptors of power quality
problems per IEC?

• Blackout
• Blink
• Brownout
• Bump
• Clean Ground
• Clean Power
• Dirty Ground
• Dirty Power

• Glitch
• Outage
• Interruption
• Power Surge
• Raw power
• Spike
• Surge
• Wink

We all need to define the power quality in proper
terms to address the problem. Is it voltage
quality, harmonics, etc? Metering and analysis
is the best way to resolve the issue.
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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.

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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.

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What Causes Harmonics?

• Non-Linear Loads
• Current is not proportional to the applied
  voltage

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Linear vs. Non-linear loads and current waveforms

• 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

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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.

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Effect of harmonics on waveform
180 Out of Phase
In Phase
When a waveform is identical from one waveform to
the next, it can be represented as a sum of pure
sine waves in which the frequency of each
sinusoid is an integer multiple of the
fundamental frequency of the distorted wave. The
sum of the sinusoids created by harmonics can be
analyzed using the Fourier series concept
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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.

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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

• Video display terminals
• File Servers
• Battery Chargers

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

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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 RMS
• Changing power system loads will vary the
  magnitude and phase angle, resulting in
  inaccurate and unpredictable sensing units and
  overload protection

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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

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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

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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

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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)

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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

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IEEE Defined Harmonic Current Limits
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Voltage Limit /HarmonicsEvaluation Procedure
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Questions?

• Thank You!
• Cornell Planning, Design Construction
• Carol Gowan 254-1457 cag57_at_cornell.edu
• Chad Loomis 255-8039 cel36_at_cornell.edu

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Simplified Distribution System Diagram
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Phase Conductors Heating from Skin
Effect Conduits Heating from Skin Effect and
eddy currents
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Simplified Distribution System Diagram
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Circuit breakers False tripping due to high
heat and higher frequencies caused by
harmonics Peak sensing CBs may trip because of
higher peaks
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Simplified Distribution System Diagram
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Neutral conductors high current from zero
sequence triplen harmonics (3rd 9th) can add as
much as 30 to neutral current even if phases are
balanced. Shared neutrals or reduced size
neutral can be hazardous (no CB to protect from
overcurrent!)
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Simplified Distribution System Diagram
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Neutral to ground voltage at receptacle
Excessive neutral current results in high voltage
drops between neutral and ground at the outlet
4
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Simplified Distribution System Diagram
5
5
Neutral bus bar overloading and heat from zero
sequence harmonics (3rd and 9th)
5
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Simplified Distribution System Diagram
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6
Panelboard enclosure heating from eddy currents
(induced currents caused by magnetic fluxes), as
well as vibration (buzzing sound).
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Simplified Distribution System Diagram
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Transformers heating from eddy currents reduces
the life of a transformer insulation system, and
lowers efficiency.
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Simplified Distribution System Diagram
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Induction Motor Leads Heating from eddy
currents and negative sequence harmonics (5th,
11th, 17th,etc.)
Note Negative sequence harmonics work AGAINST
a motors natural rotation, making it work harder!
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Simplified Distribution System Diagram
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Power factor (kVAR) Capacitors since they are
lowest impedence, resultant high harmonic current
can overheat capacitors and/or blow fuses.
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Simplified Distribution System Diagram
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kW and kVAR meter may have reading errors when
harmonic currents are present.
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Simplified Distribution System Diagram
In addition, emergency generators can have speed
control and voltage supply issues caused by
harmonics, and harmonics can impact
communications cabling and sensitive electronic
equipment.
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Typical PQ Evaluation Process