pq-abesit

8/11/2019 PQ-ABESIT

1/46

Power Quality Problems:

An Overview & Key Issues

Prof S.N. Singh

Department of Electrical Engineering,

Indian Institute of Technology, Kanpur

(Email: [email protected])

8/11/2019 PQ-ABESIT

2/46

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

8/11/2019 PQ-ABESIT

3/46

Power Quality

Quality of Supply

Quality of Service

Various Terms of Quality

8/11/2019 PQ-ABESIT

4/46

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

8/11/2019 PQ-ABESIT

5/46

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).

8/11/2019 PQ-ABESIT

6/46

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

8/11/2019 PQ-ABESIT

7/46

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 ?

8/11/2019 PQ-ABESIT

8/46

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..

8/11/2019 PQ-ABESIT

9/46

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

8/11/2019 PQ-ABESIT

10/46

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

8/11/2019 PQ-ABESIT

11/46

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

8/11/2019 PQ-ABESIT

12/46


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.

8/11/2019 PQ-ABESIT

13/46


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.

8/11/2019 PQ-ABESIT

14/46

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

8/11/2019 PQ-ABESIT

15/46

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.

8/11/2019 PQ-ABESIT

16/46

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

8/11/2019 PQ-ABESIT

17/46

8/11/2019 PQ-ABESIT

18/46

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%

8/11/2019 PQ-ABESIT

19/46

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)

8/11/2019 PQ-ABESIT

20/46

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

8/11/2019 PQ-ABESIT

21/46

Order of typical harmonics generated

by non-linear loads?

8/11/2019 PQ-ABESIT

22/46

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

8/11/2019 PQ-ABESIT

23/46

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

8/11/2019 PQ-ABESIT

24/46

Harmonic Current Limit as per IEEE-519

(customers responsibility)

SCR

=Isc/IL

h

8/11/2019 PQ-ABESIT

25/46

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)

8/11/2019 PQ-ABESIT

26/46

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

8/11/2019 PQ-ABESIT

27/46

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

8/11/2019 PQ-ABESIT

28/46

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

8/11/2019 PQ-ABESIT

29/46

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.

8/11/2019 PQ-ABESIT

30/46

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.

8/11/2019 PQ-ABESIT

31/46

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

8/11/2019 PQ-ABESIT

32/46

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

8/11/2019 PQ-ABESIT

33/46

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,

8/11/2019 PQ-ABESIT

34/46

Stationary vs. Non-Stationary

Perfect knowledge of what frequencies exist, but no information

about where these frequencies are located in time

8/11/2019 PQ-ABESIT

35/46

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.

8/11/2019 PQ-ABESIT

36/46

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

8/11/2019 PQ-ABESIT

37/46

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


8/11/2019 PQ-ABESIT

38/46

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.


Daubechies 4 Mother Wavelet

8/11/2019 PQ-ABESIT

39/46

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

8/11/2019 PQ-ABESIT

40/46

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)

8/11/2019 PQ-ABESIT

41/46

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?

8/11/2019 PQ-ABESIT

42/46

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:

8/11/2019 PQ-ABESIT

43/46

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.

8/11/2019 PQ-ABESIT

44/46

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

8/11/2019 PQ-ABESIT

45/46

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.

8/11/2019 PQ-ABESIT

46/46

Thank

You ?

pq-abesit

Documents