pq-abesit
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Power Quality Problems:
An Overview & Key Issues
Prof S.N. Singh
Department of Electrical Engineering,
Indian Institute of Technology, Kanpur
(Email: snsingh@iitk.ac.in)
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Electricity must be Economical
Stable
Reliable Secure
Good quality
Power Quality is defined as "any powerproblem manifested in voltage, current,and/or frequency deviations that results inthe failure and/or mal-operation of endusers equipment.
Electric Power
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Power Quality
Quality of Supply
Quality of Service
Various Terms of Quality
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Quality of Supply?Refers to: Supply reliability + Voltage Quality
Supply Reliability: relates to the availability ofpower at given point of system (continuity).
Voltage Quality: relates to the purity of the
characteristics of the voltage waveform including
the absolute voltage level and frequency.
QoS= Uninterrupted supply of power with
sinusoidalvoltage and current waveform at
acceptable frequency and voltage magnitude.
Quality of Service = Quality of Supply +
Customer relations
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Voltage or Power Quality
Due to Disturbances e.g. transients (switching/lightning), faults etc. (resulting in voltage sag,
swell, oscillatory and impulsive waveform,
interruption)
Due to Steady State Variations e.g. nonlinear
characteristics of loads, furnace/induction
heating loads, switching of converters etc.
(resulting in harmonics, notching and noise).
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Possible effects of poor power quality are:
Maloperation(of control devices, mains signaling
systems and protective relays)
More loss(in electrical system)Fast aging of equipments.
Loss of production
Radio, TV and telephone interferenceFailure of equipments
Effects of Poor Power Quality
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The widespread use of sensitive microprocessor-basedcontrols and power electronics devices for higherefficiency, pf improvements, adjustable speed drives etc.
The proliferation of large computer systems into many
businesses and commercial facilities; The development of power electronics equipment for
improving system stability, operation, and efficiency
(these devices are a major source of bad power qualityand are themselves vulnerable to such quality of power);
Why PQ becomes important ?
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Deregulation of power industry, which givescustomers the right to demand higher quality ofpower;
There are some indications that information about thePQ itself will become a valuable commodity afterderegulation subject to negotiations, pricing,ownership, etc
The complex interconnection of systems, resulting inmore severe consequences if any one component fails;
Huge economic losses if equipment fails or
malfunctions;
Continued..
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PQ Disturbances and their causes
PQ Disturbances
Transients
Short Duration Voltage Variations
Long Duration Voltage Variations
Interruptions
Waveform Distortion
Voltage Fluctuation (flicker)
Frequency Variation Harmonics
Main causes of poor PQ
Nonlinear loads
Adjustable-speed drives
Traction drivesStart of large motor loads
Arc furnaces
Intermittent loads transients
Lightning
Switching, transients
Faults
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Some typical PQ disturbances
Voltage Sag
Lightning Strike
Capacitor Switching
High Impedance Fault (RMS)
Harmonics
Voltage sags
Major causes:faults, starting oflarge loads, and
Major consequences:shorts,
accelerated aging, loss of data or
stability, process interrupt, etc.
Capacitor switching transients
Major causes:a power factorcorrection method
Major consequences:insulation
breakdown or sparkover,
semiconductor device damage,
shorts, accelerated aging, loss of
data or stability
Harmonics
Major causes:powerelectronic equipment, arcing,
transformer saturation
Major consequences:
equipment overheating,high
voltage/current, protective
device operations
Lightning transients
Major causes:lightning strikes
Major consequences:insulation
breakdown or sparkover,
semiconductor device damage,
shorts, accelerated aging, loss of data
or stability
High impedance faults
(One of the most difficult power system
protection problems)
Major causes:fallen conductors, trees (fail
to establish a permanent return path)
Major consequences:fire, threats to
personal safety
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Events Causes Effects Existing Solutions
Voltage
variations
Load Variations and
Switching events.
Premature ageing, Pre-
heating and malfunctioning
of connected equipment
Line voltage regulators,
UPS, Motor generator sets.
Flicker Arcing conditions,rolling mills, large
industrial motors with
variable loads
Disturbance in TV andother monitoring
equipments, light flicker.
Filters, static VAR systems,distribution static
compensators
Transients Lightning, Capacitor
Switching
Reduce life span,
insulation breakdown of
transformer and motorload.
Transient suppressors
Sag (Dip) Power System faults,
Utility equipment
malfunctions , starting
large loads and ground
faults
Malfunction of electronic
drives, converters, motor
stalling, digital clock
flashing, and related
computer system failure.
UPS, constant voltage
transformer, energy storage
in electronic equipment,
new energy-storage
technologies.
Swell SLG fault, upstream
failure, Switching of
large load, The large
capacitor bank.
Insulation breakdown of
equipments, Tripping out
of protective circuitry in
some power electronics
systems.
UPS, Power conditioner
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Events Causes Effects Existing Solutions
Harmonic
Distortion
Nonlinear industrial
loads, Variable speed
drives, Welders, Large
UPS systems, Non-linear residential loads.
Overheating and fuse
blowing of pf correction
capacitors, overheating of
neutral conductors ofsupply transformers,
Tripping of over current
protection, mal-operation
of relays
Passive and Active Filters.
Voltage
unbalance
Capacitor bank
anomalies such as ablown fuse on one
phase of a 3 bank.
Overheating of motors,
Skipping some of six halfcycles that are expected in
variable speed drives.
To reassess the allocation
of 1 phase loads from the3 system.
Interruptio
n in supply
Fault in network or by
excessively large
inrush currents,
malfunction of
customer equipment,
and fault at main fuse
box tripping supply.
loss of computer/controller
memory, equipment
shutdown/failure, hardware
damage and product loss
Energy storage in
electronic equipment,
employing UPS systems,
allowing for redundancy,
installing generation
facilities in the customers
facility.
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Events Causes Effects Existing Solutions
Under
voltage
Overloaded customer
wiring loose or
corroded connections,
unbalanced phaseloading conditions,
faulty connections or
wiring overloaded
distribution system,
incorrect tap setting
and reclosing activity.
Errors of sensitive
equipment, low efficiency
and reduced life of
electrical equipment, suchas some motors, heaters,
lengthens process time of
infrared and resistance
heating processes, hardware
damage and dimming of
incandescent lights, andproblems in turning on
fluorescent lights.
Regular maintenance of
appliance, cable and
connections, checking for
proper fuse ratings,transferring loads to
separate circuits, selecting
a higher transformer tap
setting, replacement of
overloaded transformer or
providing an additionalfeeder.
Over
Voltage
Improper application
of power factor
correction capacitors
and incorrect tap
setting.
Overheating and reduced
life of electrical equipment.
Ensuring that any pf
correction capacitors are
properly applied and
changing the transformers
tap setting.
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Categories
Typical
spectral
content
Typical
duration
Typical voltage
mag.
Transients-Impulsive -Nanosecond
-Microsecond
-Millisecond
5nsecrise
1 sec rise
0.1 msec rise
< 50 nsec 50 nsec
-1 msec
> 1msec
-Oscillatory - Low frequency- Medium frequency
- High frequency
< 5kHz
5 -500 kHz
0.5 -5 MHz
0.3 -50 msec
20 sec
5 sec
0-4pu
0-8pu
0-4pu
Short duration variations-Instantaneous
- Interruption- Sag (dip)
- Swell
- Momentary- Interruption
- Sag (dip)
-Swell
0.5 -30 cycles
0.5 -30 cycles
0.5 -30 cycles
30 cycles -3 sec
30 cycles -3 sec
30 cycles -3 sec
< 0.1 pu
0.1 -0.9 pu
1.1 -1.8 pu
< 0.1 pu
0.1 -0.9 pu
1.1 -1.4 pu
IEEE Std 1159-1995
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Long duration variations
- Interruption sustained
- Under-voltages
- Over-voltages
> 1min
> 1min
> 1min
0.0 pu
0.8 -0.9 pu
1.1 -1.2 pu
Voltage unbalance Steady state 0.5 -2 %
Wave distortiondc offset
Harmonics
Inter-harmonics
Notching
Noise
0 -100th harmonic
0-6 kHzBroadband
Steady state
Steady state
Steady stateSteady state
Steady state
0-0.1%
0 -20 %
0-2%
0.1 %
Voltage fluctuations < 25 Hz Intermittent 0.1-7%
Power frequency
variations< 10 sec
Temporary
-Interruption
- Sag (dip)
- Swell
3sec -1min
3sec -1min
3sec -1min
< 0.1 pu
0.1 -0.9 pu
1.1 -1.2 pu
Continued.
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Harmonics It is a steady state periodic phenomenon that
produces continuous distortion in voltage andcurrent waveform.
It is normally caused by saturable devices, power
electronics devices and non linear consumer loads.
Total Harmonic Distortion (THD) is a measure of
harmonic voltage/current. The THD in a voltage
waveform is defined as
where, Vnis the magnitude of nthharmonic voltage
and V1is the magnitude of fundamental voltage.
1
2
2
V
V
THD n
n
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0 0.005 0.01 0.015 0.02-1.5
-1
-0.5
0
0.5
1
1.5
Time(s)
Voltage(pu)
(a) Distorted Waveform
Fundamental
1 2 3 4 5 6 7 8 9 10 11 12 13 14 150
0.2
0.4
0.6
0.8
1
Harmonics Number
HarmonicsMagnitudes
(pu)
(b) Spectrum of (a)
Spectrum of a Typical Distorted Voltage
Waveform
THD= 43.83%
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Although easy to measure, the THD does not show
the interference impact of the signal. Total Demand Distortion (TDD) is a measure of the
THD taking into account the circuit rating. As the
circuit rating versus load current rises, TDD drops
TDD = THD x (Fundamental load current/circuit
rating)
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What do harmonics do?
Harmonics are carried through the system fromthe source and can nearly double the amount ofcurrent on the neutral conductor in three phasefour wire distribution systems.
Distorted currents from harmonic-producingloads also distort the voltage, which appear to
other end users on the system. Overall electrical system performance and power
quality is affected by the introduction ofharmonics, such as
Overheating of Transformers, Capacitors and Motors Mal-operation Relays and Circuit Breakers
Communication Interference Problems
Unreliable Operation of Electronic Equipment
Computer (PC/CPU) data errors / data loss
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Order of typical harmonics generated
by non-linear loads?
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Power Quality Related Standards of the IEEE(Recommended Pract ices)
IEEE 446 - Emergency and Standby Power
IEEE 519 - Harmonic Control
IEEE 1001 - Interface with Dispersed Generation
IEEE 1100 - Power and Grounding ElectronicsIEEE 1159 - Monitoring Power Quality
IEEE 1250 - Service to Critical Loads
IEEE 1346 - System Compatibility in Industrial
Environments
IEEE 1366 - Electric Utility Reliability Indices
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Harmonic Voltage Limit as per IEEE-519
(utilities responsibility)
Bus Voltage Maximum
Individual
Harmonic
Component (%)
Maximum
THD (%)
69 kV and below
115 kV to 161 kV
Above 161 kV
3.0%
1.5%
1.0%
5.0%
2.5%
1.5%
H i C t Li it IEEE 519
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Harmonic Current Limit as per IEEE-519
(customers responsibility)
SCR
=Isc/IL
h
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Indian standards on harmonic
limitsCBIP Report No. 251 Total Harmonic Distortion (THDV) = 9% in 0.4
< U < 45 kV
APERC
The cumulative (THDv) at the Point ofCommencement of Supply for each consumerconnected at 33kV shall be limited to 8% (asper Grid Code)
The cumulative (THDv) at the Point ofCommencement of Supply for each consumerconnected at 11kV shall be limited to 8% (asper Grid Code)
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Power Acceptability Curve
Quantify acceptability of power supply as afunction of voltage imbalance magnitude and itsduration (based on energy concept).
Originally developed by Computer BusinessEquipment Manufacturers (CBEMA) to definecapability limit of computers.
It has become standard for all types of electrical
equipments and power system. Other standard is by Information Technology
Industry Council (ITIC).
C t B i E i t
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0.0001 0.001 0.01 0.1 1 10 100 1000
-100
-50
0
50
100
150
200
250
TIME IN SECONDS
PERCENTC
HANGEINBUSVOLTAGE
8.33ms
OVERVOLTAGE CONDITIONS
UNDERVOLTAGE CONDITIONS
0.5CYCLE
RATED
VOLTAGE
ACCEPTABLE
POWER
Computer Business Equipment
Manufacturing Association (CBEMA) curve
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Information Technology Industry
Council (ITIC) Curve
0.0001 0.001 0.01 0.1 1 10 100 1000
-100
-50
0
50
100
150
200
250
TIME IN SECONDS
PERCENTCHANGEINBUSVOLTAGE
8.33ms
OVERVOLTAGE CONDITIONS
UNDERVOLTAGE CONDITIONS
0.5CYCLE
RATED
VOLTAGEACCEPT ABLE
POWER
10%+--
M it i d Miti ti f PQ
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Monitoring and Mitigation of PQ
Problems
Requires continuous and extensive monitoringof different power system quantities.
Detection and identification of power quality
related disturbances and categorizing them.
Analysis of the identified problems to their
probable causes.
Prevention and corrections of the probable
causes either automatically or manually.
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Few Chal lenges
PQ monitoring software and hardware are needed
in both utilities and customers
Detect, identify, and localize different PQ
disturbancesReal time decision making
Constraints :
Missing waveform information
There is no information about power system states.
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Continued
The current methods for detecting power quality disturbance is
based on
a point to point comparison of adjacent cycle or
a point to point comparison of the rms values of the distorted
signal with its corresponding pure signal
transformation of the data into the frequency domain via
Fourier transform (FT).
Drawbacks
It fails to detect disturbances that appear periodically,such as flat-top and phase controlled load wave shapedisturbances.
Not suitable for non-stationary signals.
Applications
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Signal Sources Power Quality Events
Transients
Short Duration
Voltage
Variations
Long Duration
Voltage
Variations
Interruptions
Waveform
Distortion
Flicker
Harmonics
Frequency
variation
transformer
inrush
Applications
statistical
analysis,
troubleshooting
relaying,
protection
incipient fault
detection
Classifiers
Expert, Fuzzy
,AI, GA
Classifiers
hidden Markov
models
PQ Analysis Stages
Matlab simulations
PSCAD/EMTDC
simulations
Standard data from
other sources
single instrument
measurement (power
platform)
Data
Compression
Feature Extractors
Wavelet with
MRA
Orthogonal
Polynomial
Approximation
Time Frequency
Representations
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Fourier Transform ( FT)
FT uses complex exponentials (sinusoids) as basis.
For each frequency of complex exponential, thesinusoid at that frequency is compared to the signal.
If the signal consists of that frequency, the correlationis high large FT coefficients.
If the signal does not have any spectral componentat a frequency, the correlation at that frequency is
low / zero, small / zero FT coefficient.
tjttje sincos
dtjeFtfdttjetfF )(
21)()()( and,
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Stationary vs. Non-Stationary
Perfect knowledge of what frequencies exist, but no information
about where these frequencies are located in time
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Short Time Fourier Transform (STFT)
STFT provides the time information by computing a differentFTs for consecutive time intervals, and then putting themtogether
Time-Frequency Representation (TFR)
Maps 1-D time domain signals to 2-D time-frequency signals Consecutive time intervals of the signal are obtained bytruncating the signal using a sliding windowing function
Wide analysis windowpoor time resolution, good frequencyresolution
Narrow analysis windowgood time resolution, poorfrequency resolution
Once the window is chosen, the resolution is set for both timeand frequency.
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Wavelet Transform (WT)
Overcomes the preset resolution problem of theSTFT by using a variable length window
Analysis windows of different lengths are used fordifferent frequencies:
Analysis of high frequenciesUse narrowerwindows for better time resolution
Analysis of low frequencies Use widerwindows for better frequency resolution
This works well, if the signal to be analyzed mainlyconsists of slowly varying characteristics withoccasional short high frequency bursts.
The function used to window the signal is called the
wavelet
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Take a wavelet and compare it to a section at the start of theoriginal signal.
Calculate a correlation coefficient c(i.e. Assign a coefficient
of similarity )
Low scale: aCompressed wavelet
Rapidly changing details
High frequency
High scale:
aStretched wavelet
Slowly changing, coarse
features
Low frequency
Wavelet Transform (WT)
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3. Shift the wavelet to the right and repeat steps1 and 2 until the whole signal is covered .
4. Scale (stretch) the wavelet and repeat steps
1 through 3.
5. Repeat steps 1 through 4 for all scales.
Wavelet Transform (WT)
Daubechies 4 Mother Wavelet
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D-Wavelet Transform Fundamental concepts of DWT
Provides time-scale (frequency) representation of non-stationarysignals
Based on multiresolution approximation (MRA) Approximate a function at various resolutions using a scaling
function, (t)
Keep track of details lost using wavelet functions, (t) Reconstruct the original signal by adding approximation and
detail coeff.
Implemented by using a series of lowpass and highpassfilters
Lowpass filters are associated with the scaling function andprovide approximation
Highpass filters are associated with the wavelet function andprovide detail lost in approximating the signal
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Dyadic sampling of the timefrequency plane results in a very
efficient algorithm for computing DWT:
Dyadic sampling and multiresolution is achieved through a series of
filtering and up/down sampling operations
Multiresolution analysis
Discrete Wavelet Transform (DWT)
h(n)
g(n)
2
2
c1(n)
c0(n)
h(n)
g(n)
2
2
c2(n)
d1(n)
d2(n)
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Multiresolution Analysis (MRA)
x[n] B: 0 ~
g[n] h[n]
g[n] h[n]
g[n] h[n]
2
d1: L evel 1DWT
Coeff.
B: 0 ~ /2 Hz
d2: Level 2
DWTCoeff.
d3: L evel 3
DWT
Coeff.
.
2
2 2
22
B: 0 ~ /4 Hz
B: 0 ~ /8 Hz
In DWT, only approximationcoefficients are decomposed.
Each decomposition
allows dyadic dichotomization
of the frequency spectrum
What if we were decompose the
detail coefficients as well?
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Wavelet Packets
x[n] B: 0 ~
H G/2 ~0 ~ /2
G3/4 ~
H/2 ~ 3/4
G/4 ~ /2
H0 ~ /4
H0 ~ /8
G/8 ~ /4
H/4 ~ 3/8
G3/8 ~ /2
H/2 ~ 5/8
G5/8 ~ 3/4
H3/4 ~ 7/8
G7/8 ~
AAA(3) DAA(3) ADA(3) DDA(3) AAD(3) DAD(3) ADD(3) DDD(3)
AA(2) DA(2) AD(2) DD(2)
A(1) D(1)
H H 2:
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Other Key Issues and Challenges
PQ Event generation for testing the tools
PQ Measurement Locations
Noise in PQ captured data
Adaptive filtering due to changing
Selection of model order for detection scheme
Choice of different mother wavelets
Development of AI/DSP tools to classify events
Economic evaluation of PQ problems.
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Role of Regulators
Set general quality targets at different levels Comparison of Standards
Monitor quality levels
Penalties for not respecting quality standards Financial compensation scheme
Dispute settlement procedures
Quality of supply in changing environment
Conclusions
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Conclusions Quality of Supply is an important issue to be
addressed by utilities as well as customers at
transmission & distribution levels. Indigenous QoS standards should be developed based
on the techno-economic analysis.
To improve the QoS, network and generation
capabilities must be enhanced. Power quality problems have been classified in
different forms. Its analysis and classification is very
important.
Proper monitoring of PQ signals, its analysis toidentify the type of the PQ problem, its impact analysis
and installing different types of mitigation devices in
the system are the main steps in addressing the PQ
problems.
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Thank
You ?
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