Transcript basics_of_power_qual..

Understanding
Power Quality Problems
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Objectives
• This presentation has been adapted to NJATC
Lesson 4-62. It’s intent is to give the student a
basic understanding of Power Quality as describe
in this lesson plan.
– Describe the nature of several different types of powerrelated problems.
– Identify the causes or source of many power related
problems
– List steps that can be used with different electrical
equipment to minimize the equipment’s contribution to
power-related problems.
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What is Power Quality
Except for complete failure, most really don’t pay
close attention to the power we are supplied.
But if we were in the middle of an important
document or key manufacturing process it would
be more than a simple announce.
A knowledge of power quality is necessary to
identify and resolve electrical environmental
problems
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What is Power Quality
With the emergence of the “Electronics Age”
there has be a change in the requirement of
our electrical distribution systems.
Unlike traditional load, like lighting or
motors, Sensitive loads are more
susceptible to power disturbances.
Electronic equipment requires a much more
stable power source.
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Importance of Power Quality
• Consequences of poor Power Quality can result in:
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Lost productivity
Lost/corrupt data
Damaged equipment
Poor power efficiency
• U.S. companies waste an estimated $26 billion on
electrical power-related issues each year*
*Electrical Contractor Magazine, “Surveying
Power Quality Options” March 2000
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Power Quality
• There is no absolute definition of power quality, but many
define it as the degree to which both the utilization and the
performance of electric power affects the performance of an
electrical distribution system.
• Organizations such as the IEEE -Institute of Electrical and
Electronics Engineer and ANSI -American National
Standards Institute are setting Stringent requirements for
power Quality.
– IEEE 519-81 Recommended limits on Harmonics
– ANSI C84.1 Specification on normal voltage ratings and tolerances
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Key Contributors
• Power Factor
– Measure of how efficiently Power is used
• Disturbances
– Momentary disruptions to the Electrical system
• Harmonics
– Integers of the Fundamental Frequency which have an
effect on the electrical systems and loads
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Power Quality
• The NIST -National Institute of Standards and
Technology publication NIST SP768 Shows that
power disturbances can be defined in two
categories, Steady State or Intermittent.
– Steady state disturbances are Noise, Harmonics, long
term Overvoltage or Undervoltage conditions.
– Intermittent disturbances are Sages, Swells, Impulse,
Transients, and interruption,
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ITI (CBEMA) Curve
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Power Factor
– Incandescent lighting,
heating elements,
motors and capacitors
– Affect on Electrical
source are phase shifts
due to capacitive and
inductive reactance
– Phase shift is
measured as Power
Factor or
PF
Most utilities have an additional charge
if PF is less than .95 to .90
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Power Factor
• Apparent Power is the product
of Volts times Amps
– VA or KVA=V x A
• Real Power is the time average
of instantaneous product of Volts
time Amps
– W or kW = V x A x cosine theta
•
• Time average of
instantaneous evaluates
phase shift between Volts
and Amps
Power factor is Watts divide by
Apparent Power
–
PF= W/VA
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Power Factor
• VAR or Reactive Power
is magnetic energy, which
causes a phase shift
between voltage and
current Waveform
• This overlapping of
waveforms delivers less
power to the load
– VA2=VAR2 + W2
– VAR =
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VAR’s
• VAR’s or the non-working
energy may be thought of
as water in a hose.
• When water is first
applied there is a slight
delay before water gets to
the end of the hose.
• If we used a storage
device, water at the end of
the hose would flow
immediately.
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Power Disturbances
• Power disturbances generally fall into one
of six categories.
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Voltage Sags and Swells
Under-voltage or Over-voltage
Transients spikes, impulses and surges
Outages
Harmonics
Noise
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Power Disturbances
• Voltage Sag
– Momentary decrease in line Voltage
– .5 to 30 Cycles (up to .5sec)
– Caused by the start of heavy loads or
fault occurrence on source.
• Voltage Swells
–Momentary increase in line Voltage
–.5 to 30 Cycles (up to .5sec)
–Occurs due to sudden load decrease or
de-energizing of heavy equipment.
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Power Disturbances
• Over-Voltage
– Abnormally high voltage
– >.5sec to a few seconds
– Power voltage regulation
• Under-Voltage
– Abnormally low Voltage
– >.5sec to a few seconds
– Result for clearing of a fault or
intentional utility regulation.
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Power Disturbances
• Transients:
– Short duration high
amplitude pulses are surges
that are superimposed on a
normal voltage waveform.
– Vary widely from twice the
normal voltage to several
thousand volts in time
from < microsecond to a
few hundreds of a second.
– Result for loads cycling on
and off in a building,
utility, or lighting
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Power Disturbances
• Outages
– Outage is a complete loss
of power lasting from a
few milliseconds to several
hours.
– Caused by power system
failure due to damage to
supply lines or equipment
failure
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Power Disturbances
• NOISE: is an unwanted
signal or distortion that is
superimposed on a
normal voltage waveform
– Normal Mode noise
– Common Mode noise
– RFI: Radio Frequency
interference
– EMI: Electromagnetic
interference
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Harmonics
• Harmonics are multiples of the fundamental
frequency of 60 Hz in an electrical system.
• Most harmonics are generated from Non-linear
loads, or solid-state device equipment
– Examples are Computers, Copiers, Laser printers,
UPS systems, industrial controls, welding machines,
Adjustable Speed Drives {ASD} or Variable Speed
Drives {VFD}
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Harmonics
• Non-linear is used to
describe the switch
mode power supply
{smps} found in most
microprocessor based
equipment and
rectified supplies
found in industrial
loads.
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Harmonics
• Non-Linear loads
– Computers, printer,
copiers, electronics lighting
– Adjustable speed drives
and other microprocessor
controlled equipment
– Effects on electrical
systems or odd harmonics
3rd, 5th, 7th etc.
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Harmonics
• Total Harmonic Distortion
– Expressed as %THD
– Percentage of distortion to the sine wave
– Should not exceed 5% of line voltage or 20% of current
• Harmonic FFT’s
– Breakdown of the THD to the individual harmonics
– Show the amount of harmonic as a percentage of the
fundamental.
Knowing each harmonic and its effect can help in determines
the impact on the system
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Plan, Investigate & Test
• Where to start
– Plan your site survey
– Investigate suspected
areas
– Test or monitor
– Analyze results or date
Please refer to Table 2
Electrical Distribution Systems Equipment and
Grounding Measurement and Considerations
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Plan
Make a block diagram of you facility.
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Investigate
• Investigate suspected
areas
• Try to establish time of
occurrence and duration
history
• Equipment usage cycles or
new equipment instillation
• Personnel
• Interview others- Find out
what they have observed
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Impulse
• Most impulses are generated within a
facility when inductive loads are switched
on and off
• Typical causes of impulses include switch
contacts and sharp current transitions
interacting with source impedance.
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Impulse
• The origin of an impulses can
be determined by reviewing
the polarity of the leading
edges of the simultaneous
voltage and current impulses
• If the voltage is “+” and
current is “+”, or if the voltage
is “-” and the current is “-”
then the origin is sourcerelated.
• If the voltage is “+” and the
current is “-” then the origin is
load related.
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Voltage Sags
• Voltage sags are sometimes
caused by a problem on the
utility power supply outside of
the facility
• If current on a circuit being
monitored increases during a
sag the origin is normal load
related.
• If current decreases or drops to
zero during a sag the origin is
normal source related.
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Voltage Sags
• Other causes are, load interaction
with wiring, (equipment at start-up)
or voltage source impedance
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Voltage Distortion
• Voltage Distortion can be caused
by large harmonic currents from
Nonlinear loads or Power sources
with no sinusoidal Voltage
Characteristic
• Linear loads have small effects on
voltage distortion.
• Non-Linear loads have a larger
effect on voltage distortion
Excessive Current drawn as the Voltage waveform
reaches Peak can cause Voltage distortion. Referred
to as Flat Topping or Clipping.
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Ground to Neutral Events
• Causes of neutral-toground voltage
– Return current in
neutral conductor
– Excessive neutral
conductor resistance
– Ground current
– Excessive ground
resistance
Ohm’s Law we can derive Voltage between Neutral
And the ground. V=I * R, V= 50A * .1 ohm or V=5
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Where to Look
• Most power quality failures may be tracked
down to one of three areas.
– Supply- Utilities and its distribution.
– Internal Distribution- Feeders and Branches,
Grounding, wiring and termination
– Internal Loads- load disturbances and
Harmonics
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Supply- Utilities disturbance
• Supply Disturbances and
service area
– Utility faults,
• switching transients,
regulation
– Lighting,
• Transients, outages
– Accidents to Transmission
lines
• outages
– Failure of backup sources
• Outage, under of over
voltages
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Internal Distribution
• Inspect your Electrical distribution entrance
service, ensure compliance with NEC.
• Look for:
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Check ground
Corroded connections
Defective conduit
Defective electrical devices
Adequate wiring
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Internal Distribution
• Inspect all grounding, ensuring compliance with
NEC.
– Inspect receptacle, sub-panels, Feeder and etc, for
proper grounding.
– Inspect neutral to ground bonding per NEC
compliance
• Inspect Electrical panels.
– Loose electrical connections, improper neutral to
ground bonding at sub-panels.
– Voltages phase to neutral , phase to ground, and phase
to phase
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– Current in branch
andof Power
feeders.
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Internal Loads
• Identify major Loads
– Computers
– Copiers, Laser printers, and
other Large office loads
– HVAC Equipment
– Industrial equipment, Like
ASD’s (Adjustable
Speed Drives)
– Lighting
– UPS systems
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Internal Loads
• Transient
– Inductive loads
• Sags
– Starting of large loads
• Swells
– Large load removed
• Harmonics
– Caused by solid-state
electronics
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Effects of Harmonics
Commercial
• Effects of Harmonics
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Heating of Transformers
Heating of neutrals
Hot breakers and panels
High Voltage drop or Flat Topping
High Neutral to ground Voltage
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Effects of Harmonics
Industrial
• Effects of Harmonics
– Heating of Transformers
– Effects on Electromagnetic equipment
– Power Factor Capacitors problems
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Transformers
• Cause of Transformer heating
– Harmonic currents cause an increase in copper
losses and stray flux losses.
– Harmonic voltage cause an increase in iron
losses
– Combined effect on transformer is heating
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Transformer
• K-Factor rated transformers were designed to handle
the additional heat generated by high harmonics.
• Recommended Practice for Establishing Transformer
Capabilities,
– IEEE Standard C57.110-1986,
• Never use a K-Factor that is higher than needed. It
will reduce the transformers ability to withstand
power glitches.
– K rated transformers have lower impedance. You need
some impedance to reduce effects of disturbances.
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De-rating Transformers
• This method should only be used in de-rating
phase to neutral loads.
– Measure the True rms current for each phase
– Measure the instantaneous peak for each phase
– If the Ipeak and Irms values on each phase are not close,
use the average value for each.
– THDF= (1.414 x Irms)/ Ipeak.
• (a value between 0 and 1.0)
– KVA de-rated=KVAnameplate x THDF
• (avg de-rating of a transformer is in the range of 20% to 40%)
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Hot Neutral Conductors
• Neutral Conductors can have as much as 150% of
triplen harmonics (3rd, 9th, 15th, etc) as any phase
conductor.
– Triplen harmonics don’t cancel, but add in the neutral
conductor.
– “Skin Effect”, higher frequencies travel not through
the wire, but on the outer surface of the wire.
• Small gauge wire have less surface so it is a higher resistance
to the higher frequencies currents
• This is one reason that conduit makes such a good ground for
noise- the higher frequencies run on the surface of the
conduit.
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Hot Neutral Conductors
• Reducing the effects of hot neutrals
– One is to up-size the neutral. In resent years this has
been the most popular action.
– Triplen harmonic filters could be used at key source
panels.
• This would reduce not only neutral currents but also help
reduce phase to neutral load on transformers.
– No shared neutrals.
• In a 240/120 three phase distribution shared neutrals were
common, but with so many SMPS being added to the
electrical distribution performance wiring practice can reduce
effect on power disturbances.
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Branch Circuits
• Line to Neutral Voltage
– Transients, Sags& Swells,
Voltage drops, Flat toping
• Neutral to Ground Voltage
– Tripling harmonics, High
ground impedance.
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Branch Circuit
• Test receptacles in sequence along the branch circuit
• Identify the receptacle where the voltage drop increases
significantly
• Check voltage drop on remaining receptacles
– If voltage drop is acceptable in remaining receptacles, then problem
is probably localized at receptacle connection
– If voltage drop is unacceptable, then problem exists within the hot
or neutral conductors
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Integrity of the Branch Circuit
• If all branch receptacles have unacceptable
voltage drops, then the problem is most
likely in the circuit between the panel and
the first receptacle, or at the panel
– Undersized wire for length of run
– Possible splices
– Poor connections or corroded contacts at panel
• Breaker, neutral bus, etc.
– Most show up as hot spots in the panel
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Integrity of the Branch
Circuit
• Ground to neutral voltage measurement
– Indicates feedback on the neutral conductor
– Caused by load balance or harmonic
• Three-phase 208/120V power systems
– 2 volts or less is acceptable level of voltage
– IEEE recommends 0.25 ohms or less for any phase
conductor, Ground is not considered a conductor by NEC
however it would be a good guideline to consider.
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Integrity of the Ground
• Identify false grounds
– False or “bootleg” grounds are defined as a
short between neutral and ground
– Accidental short or improper bonding of
ground and neutral conductors
– Shows up as normally wired condition with
receptacle testers
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Installation of Safety Devices
• Ground Fault Circuit Interrupters
– NEC requires installation of GFCI in
bathrooms, kitchens and outside.
– Purpose is to protect individuals by detecting
ground faults.
– Defective or improperly installed GFCI can
lead to shock.
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Installation of Safety Devices
• TVSS Transit voltage surge suppresser
– Purpose is to protect equipment.
– 70% of all surges are generated within a facility
– Defective TVSS can lead to lost productivity
and damaged equipment.
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Recommend Practices
• Limit the number of electronic loads on
sensitive circuit
– Recommendation: limit the number of outlets
to 3 to 6, instead of 13 or 14
– Not practical for all wiring installations, but
essential in areas with high density of
electronic equipment
– Place large loads on dedicated circuit
– Limits the harmonic distortion on the branch
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Recommend Practices
• Neutrals, double-sized or larger on 3-phase
systems. (Most common 208/120 3 phase/4wire)
• Phase conductors, upsized to reduce
voltage fluctuations
• Use harmonic rated transformers or de-rate
standard transformers
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Recommend Practices
• Install isolated grounds to protect costly or
vital electronic equipment from harmonic
distortion
– Computers, ATM machines and expensive
copiers often require isolated grounds
– Important issue in hospital environment
• Verify isolated ground before installation of
electronic equipment
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Recommend Practices
Multiple Circuit – Separate Neutral - Loop Feed
(w/Pigtail Connections)
•
•
•
•
Limit the number of outlets
Place larger loads on dedicated circuits
Run separate neutrals (no shared neutrals)
Isolated Grounds
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Industrial Concerns
• Power Factor
– Cause by our inductive loads
• Sags and Swell; over or under voltage
conditions
– Cause heavy loads or source variations
• Phase voltage and current balance
– Source or load problems
• Harmonics and harmonic resonance
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Industrial Concerns
• Power factor
improvement can
reduce supply need
and cost.
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Industrial Concerns
• Sags, Swells, over-voltage, under Voltages
– Plus or Minus 10% will trip in most drives
• Voltage Balance
– Plus or Minus 2% to 5% can cause problem in most
drives
• Current Balance
– Plus or Minus 20%, 10% or less or Adjustable speed
drives are more sensitive to Imbalance. Consult with
drive or motor manufactures for acceptable imbalance
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Motor Voltage Imbalance
• First measure the
three phase Voltages
– V average {Vavg.}
• Vavg.=V Total/3
– V deviation {Vdev.}
• Vdev=V max/V avg.
– V unbalance(Vub)
• Vub=Vdev/V avg *100
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460V
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L1-L3
481V
L2-L3
475V
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Motor Current Imbalance
• First measure the three
phase Currents
– A average {Aavg.}
• Aavg.=A Total/3
– A deviation {Adev.}
• Adev=A max/A avg.
– V unbalance(Vub)
• Vub=Vdev/V avg *100
A phase B phase C phase
30 A
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28 A
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Industrial Concerns
• Commonly found in industrial facilities are
– Variable Frequency drives {VFD’s}
– Variable Speed Drive {VSD’s}
– The most common is the pulse width
modulation drive {PWM}
• Harmonic reversal {5th , 11th, and etc}
• Resonance Harmonic migration to Capacitor Banks
• Transient voltage spikes at motor caused by
improper installation of drive
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Reference Material
• Reference and additional Information
– IEEE Emerald book, Recommended Practice for Powering
and Grounding of Sensitive Electronic Equipment.
– Federal Information Processing standard Pub #94 ie
{FIPS-PUB-94]
– EC&M Practical Guide to Quality Power for Sensitive
Electronics Equipment 2nd Edition.
– Electrical Power Research Institute , Wiring and
Grounding for Power Quality
– Copper Development Association, A Primer on Power
Quality.
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Test and Monitor
• Test or Monitor , Basic
measurement tools
– Circuit Analyzer
– Multimeter or
ClampMeters
• (True RMS responded)
– Receptacle Event Recorder
– Power Quality Monitor.
– Infrared temperature
device
Always follow proper Safety precautions
Lock-out Tag-out, safety gear like glasses and gloves
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Test and Monitor
• Most equipment, like circuit Analyzers,
meters and clamps are slow responding.
– They sample at speeds around 2 or 3 times a
second.
– OK for steady state problems
– To slow for intermittent disturbances.
– Intermittent disturbances require equipment
which samples at a number of samples per
cycle.
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Instrumentation
• True RMS
– Digital Mulitmeters
– ClampMeter
– Accessories rated with higher
Frequency bandwidth.
• Avg. responding instruments
can have an error of as much
as 20% to 40% when
measuring Non-sinusoid
waveforms
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Two Methods of
Measuring AC
• Average responding RMS
Calibrated
– Meter responds to the average
value of a sinusoidal waveform
and multiplies it by 1.11 to
display the RMS result.
– Works well only for “pure sine
wave” or “sinusoidal wave
form”
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Two Methods of
Measuring AC
• True RMS responding
– The Meter calculates the true
effective (heating) value of
the waveform.
– Average sensing meters will
not correctly display the
effective (heating) value on a
“non-sinusoidal waveform.
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Two Methods of
Measuring AC
• In this example, the error of
the avg. reading is –35%.
• Avg. meters used to measure
non-sinusoidal wave forms
can have an error of +10% to
–40%
• Always use a True RMS
instrument whenever you are
in an electrical environment
with non-liner loads.
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Troubleshooting Equipment
• 800 Series Power Clamps
• SureTest Circuit/Harmonics Analyzers
• 800 Series Power Analyzer
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SureTest
Circuit/Harmonics Analyzers
• Circuit Analysis
– Measures voltage drop
under full 15 Amp load,
ground impedance
• Power Measurements
– Power Factor, kW
• Harmonic Measurements
– %THD, harmonic
factorization to 31st
harmonic
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800 Series Power Clamps
• Power Measurements
–
–
–
–
True Power (kW)
Power Factor (PF)
Apparent Power (VA)
Reactive Power (kVAR)
• Dual display to view key
measurements
simultaneously
• Calculates power
measurements on threephase systems
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800 Series Power Analyzer
• Power measurements
– kW, Power Factor, kVA,
kVAR
• Harmonic measurements
– %THD, harmonic
factorization to 51st
harmonic
• Disturbances
– Capture level 2 transients
(0.5µs)
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