Title: Power Quality and Harmonics: Causes, Effects and Remediation Techniques Carol Gowan Chad Loomis, PE
1Power Quality and HarmonicsCauses, Effects and
Remediation Techniques Carol
Gowan Chad Loomis, PE Cornell
University PDC 12/13/2006 Electrical Design
Section
2Overview
- 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
3What 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
4What 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
5Categories and Characteristics of Power System
Electromagnetic Phenomena (IEC)
6Voltage 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.
7Voltage 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
8Voltage 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
9Voltage Fluctuations withinCornells Utility
Distribution
- Harmonic Distortion
- Typically generated within a facility, not a
distribution issue - Utilities does not protect for this condition
10CBEMA 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
11ITIC 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
12Computer Equipment Disturbance Table(Dranetz
-BMI Field handbook for PQ Analysis)
13What 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.
14What 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.
15What 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.
16What Causes Harmonics?
- Non-Linear Loads
- Current is not proportional to the applied
voltage
17Linear 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
18Linear 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.
19Effect 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
20What 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.
21Sources 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
22What are the order of typical harmonics generated
by non-linear loads?
23Effects 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
24Effects 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
25Effects 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
26How 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
27How 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
28How 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)
29Computer 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
30IEEE Defined Harmonic Current Limits
31Voltage Limit /HarmonicsEvaluation Procedure
32Questions?
- Thank You!
- Cornell Planning, Design Construction
- Carol Gowan 254-1457 cag57_at_cornell.edu
- Chad Loomis 255-8039 cel36_at_cornell.edu
33Simplified Distribution System Diagram
1
1
Phase Conductors Heating from Skin
Effect Conduits Heating from Skin Effect and
eddy currents
1
34Simplified Distribution System Diagram
2
Circuit breakers False tripping due to high
heat and higher frequencies caused by
harmonics Peak sensing CBs may trip because of
higher peaks
2
35Simplified Distribution System Diagram
3
3
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!)
3
36Simplified Distribution System Diagram
4
Neutral to ground voltage at receptacle
Excessive neutral current results in high voltage
drops between neutral and ground at the outlet
4
37Simplified Distribution System Diagram
5
5
Neutral bus bar overloading and heat from zero
sequence harmonics (3rd and 9th)
5
38Simplified Distribution System Diagram
6
6
Panelboard enclosure heating from eddy currents
(induced currents caused by magnetic fluxes), as
well as vibration (buzzing sound).
6
39Simplified Distribution System Diagram
7
8
7
8
Transformers heating from eddy currents reduces
the life of a transformer insulation system, and
lowers efficiency.
7
8
40Simplified Distribution System Diagram
9
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!
9
41Simplified Distribution System Diagram
10
Power factor (kVAR) Capacitors since they are
lowest impedence, resultant high harmonic current
can overheat capacitors and/or blow fuses.
10
42Simplified Distribution System Diagram
11
kW and kVAR meter may have reading errors when
harmonic currents are present.
11
43Simplified 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.
44Typical PQ Evaluation Process