Power QualityPower Quality

M.JegadeesanM.JegadeesanASP/EEEASP/EEEKLN College of KLN College of EngineeringEngineeringPottapalayamPottapalayam

Electricity

• Electricity provides the power you need to run your home electronic equipment.

• Sometimes, interference in the supply of electricity affects your equipment runs.

• Many older appliances can tolerate short power disturbances.

• Many newer appliances, such as personal computers, Microwave ovens and sophisticated stereo systems, have sensitive electronics that can be disrupted or damaged.

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Electricity

Utilities transmit electricity over power lines and into your home as an alternating current (AC) wave, which looks something like an electrocardiogram. This is how power travels through your wiring and passes into your appliances.

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Power quality

Our Power Systems are designed for

Now the power system serves

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What is Power Quality?

• ‘Power quality’ broadly refers to the delivery of a sufficiently high grade of electric service.

• In general, it involves maintaining a sinusoidal load bus voltage at stipulated magnitude and frequency.

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Why is Power Quality Important?

• It affects both utilities as suppliers and customers as

users

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Impact on Customer Side

• Computers and communication equipment are susceptible to power system disturbances which can lead to loss of data and erratic operation.

• Automated manufacturing processes such as paper-making machinery, chip-making assembly lines, etc. can shutdown in case of even short voltage sags.

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Impact on Customer Side (cont.)

• Induction and synchronous motors can have excessive losses and heating.

• Home electronic equipment are vulnerable to power quality problems - e.g., blinking VCR machines and digital clocks.

• Equipment and process control malfunction translates to dollars of expense for replacement parts and for down time, impacting adversely on profitability and product quality.

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Impact on Utility Side

• Failure of power-factor correction capacitors due to resonance conditions.

• Increased losses in cables, transformers and conductors, especially neutral wires.

• Errors in energy meters, which are calibrated to operate under sinusoidal conditions.

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Impact on Utility Side (cont.)

• Incorrect operation of protective relays, particularly in solid-state and microprocessor-controlled systems.

• Interference with ripple control and power line carrier systems used for remote switching, load control, etc.

• Unhappy customers as well as malfunction and failure of system components and control systems, impacting adversely on profitability.

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Sources of Power Quality Problems

• Power electronic devices• IT and office equipments• Arching devices• Load switching• Large motor starting• Embedded generation• Sensitive equipment• Storm and environmental related damage

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Common Manifestations of Power quality

Reactive power - Low power factorHarmonics - current & voltage distortionsFrequency limits - under & over frequenciesSteady state voltage limits - under & over voltagesTransientsSags & SwellsUnbalanceSequence componentsBlack outs & Brown outs FlickerNeutral shifts

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Several typical PQ disturbances

POWER QUALITY

What do we need ? What do we have ?

Culprits Victims

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POWER QUALITY

?Power Quality issue is defined as "any occurrence manifested in Power Quality issue is defined as "any occurrence manifested in

voltage, current or frequency deviation that results in damage, upset, voltage, current or frequency deviation that results in damage, upset,

failure or malfunction of end use equipment".failure or malfunction of end use equipment".

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Power quality problems cancause: Equipment malfunctions Excessive wear or premature failure of equipment Increased costs from downtime Increased maintenance, repair time and expense Outside consultant expense

Why is it a Concern?

POWER QUALITY

• Power quality is the combination of Voltage Quality and Current Quality.

Voltage Quality• Voltage Quality is concerned with

deviation of the actual voltage from the ideal voltage.

• The ideal voltage is the single frequency sine of constant frequency and amplitude.

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POWER QUALITY

Current Quality

• Current Quality is concerned with deviation of the actual Current from the ideal Current.

• The ideal current is the single frequency sine of constant frequency and amplitude.

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Pure resistance, inductance, and capacitance are all linear.

a specific value of ohms, the relationship of volts and amperes is a straight line.

A linear element in a power system is a component in which the current is proportional to the voltage. In general, this means that the current wave shape will be the same as the voltage

Example: Incandescent lighting, heating loads, and motors

Linear vs. Non-linear loads

Linear

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The current wave shape on a non-linear load is not the same as the voltage.

These loads do not exhibit a constant impedance during the entire cycle of applied sinusoidal voltage.

Examples of non-linear loads

In single phase Computers, Fax Machines,

Photocopiers, UPS’s, TV’s, VCR’s, Lighting dimmers & Electronic ballasts for high efficiency lighting Single-phase AC & DC drives, Ultra-violet disinfection systems.

Three Phases Variable speed AC & DC drives,

UPS systems, Arc furnaces & SCR temperature controllers, Battery chargers, etc.

Non-Linear

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POWER QUALITY PROBLEMS

Overloading,

under voltage

sustained interruption

Waveform Distortion Harmonics, Noise & Interference

Voltage Fluctuations Voltage Sags & Swells

Steady state Disturbance

Long Duration Disturbances Overvoltage Under voltage Sustained InterruptionsShort Duration Disturbances Sag Swells Interruptions

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Long Duration Disturbances• Long-duration variations encompass root-mean-

square (rms) deviations at power frequencies for longer than 1 min.

• Long-duration variations can be either over voltages or under voltages.

• Overvoltages and undervoltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations.

• Such variations are typically displayed as plots of rms voltage versus time.

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Overvoltage An overvoltage is an increase in the

rms ac voltage greater than 110percent at the power frequency for a duration longer than 1 min.

• load switching (e.g., switching off a large load or energizing a capacitor bank).

• Incorrect tap settings on transformers can also result in system overvoltages.

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Under voltage An undervoltage is a decrease in the rms ac voltage

to less than 90 percent at the power frequency for a duration longer than 1 min.

opposite of the events that cause overvoltages. load switching on or a capacitor bank switching off can

cause an under voltage

Brownout The term brownout is often used to describe sustained

periods of undervoltage initiated as a specific utility dispatch strategy to reduce power demand.

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RMS Measurement of under voltage during one day

Sustained Interruptions

• When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption.

• Voltage interruptions longer than 1 min are often permanent and require human intervention to repair the system for restoration.

Outage • Utilities use outage or interruption to describe

phenomena of similar nature for reliability reporting purposes. However, this causes confusion for end users who think of an outage as any interruption of power that shuts down a process.

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Short Duration Disturbances

Voltage sag

Voltage sag is a reduction in RMS voltage at the power frequency for duration of 0.5 cycles to 300 cycles.

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Typical end-use equipment sensitive to voltage sags are: computers, programmable logic controllers, controller power supplies, motor starter contactors, control relays and adjustable speed drives.

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Voltage Sag

Sags can cause computers and other sensitive equipment to malfunction

or simply shut off. Undervoltage conditions can damage certain types of electrical equipment.

Voltage swell

Voltage swell is an increase in RMS voltage at the power frequency for duration of 0.5 cycles to 300 cycles.

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Voltage Swell

Short interruption

Short interruption is the complete loss of the supply voltage with in

a time period of 0.5 cycles up to 150 cycles.

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Voltage Variation Sources

Supply side variations Short circuits Capacitor switching Load switching Regulator malfunction Load side variations Motor starting

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Voltage Flicker• A waveform may exhibit voltage flicker if its

waveform amplitude is modulated at frequencies less than 25 Hz, which the human eye can detect as a variation in the lamp intensity of a standard bulb.

• Voltage flicker is caused by an arcing condition on the power system.

• Flicker problems can be corrected with the installation of filters, static VAR systems, or distribution static compensators

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Example voltage waveforms showing flicker created by an arc furnace

POWER QUALITY PROBLEMS

Harmonic distortion

Harmonic distortion is the periodic deviation of the voltage or current

from the ideal sinusoidal waveform, which have frequencies of

multiple integral of the fundamental frequency.

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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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Harmonics

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• The base frequency of the power supply is said to be the fundamental frequency or first harmonic.

• The fundamental frequency or first harmonic of a 60 Hz power supply is 60 Hz.

• Additional harmonics can appear on the power supply. These harmonics are usually whole number multiplies of the first harmonic.

• The third harmonic of a 60 Hz power supply, for example, is 180 Hz (60 x 3).

Harmonics

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• When a harmonic waveform is superimposed on the fundamental sine wave a distinctive waveform is produced.

• In this example, the third harmonic is seen superimposed on the fundamental frequency. The problem of waveform distortion becomes more complex when additional harmonics are present.

Harmonics

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Resultant wave form

Harmonic Distortion

Harmonics are multiples of the fundamental frequency

(5 )( )

5

Sin Xf X

+ =

f(x) = sin(x)

(5 )( ) ( )

5

Sin Xf X Sin X

When added together

result in a distorted waveform

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Distorted Waveform Composed of Fundamental and 3rd

Harmonic. THD approximately 30%

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Effect of harmonics on waveform

In Phase

180 Out of Phase

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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

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Interharmonics are defined as frequency components of voltages or currents that are not an integer multiple of the normal system frequency (e.g., 60 or 50 Hz).

The main sources of interharmonics are static frequency converters, cycloconverters, induction motors, and arcing devices. Power line carrier signals can be considered as interharmonics. The effects of interharmonics are not well known but have been shown to affect power line carrier signaling and induce visual flicker in display devices such as cathode ray tubes (CRTs).

Interharmonics

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Common Power System Harmonics In Bold

Square Wave Harmonic Content

Fund

3

3,5

3,5,7

3,5,7,9

3,5,7,9,11

3,5,7,9,11,13

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Practical Motor Drive CircuitsAlmost all motor drive circuits consist of three parts:A input converter to change the AC to DC;A DC link to store and filter the DC;An output inverter to change the DC into AC.Both output voltage and frequency must be controlled together for motor load.

AC-DCConversion

DC-ACConversion

DC Link

AC Input;fixed Frequency,fixed Voltage

AC Output;variable Frequency,variable Voltage

MotorCapacitor orInductor

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Point of Common Coupling

• The point of common coupling is the location in the power distribution system where harmonic distortion is to be measured, usually where harmonic currents flow into a bus which feeds other equipment. Its location must be specified!

• In the absence of a specified location, the POCC for current harmonics is the plant-utility interface

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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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Noise refers to unwanted electrical signals (with broadband spectral content lower than 200 kHz) that produce undesirable effects in the circuits of control systems in which they Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment, loads with solid-state rectifiers, and switching power supplies. Noise problems are mainly caused by improper grounding.

There are two types of noise voltages:

Common-mode noise voltage:

A noise voltage that appears between current carrying conductors and ground. That is, this noise voltage appears equally and in phase from each current-carrying conductor to the ground.

NOISE

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Normal-mode noise voltage:

A noise voltage that appears between or among active circuit conductors, but not between the grounding conductor and the active circuit conductors.

Noise disturbs electronic devices such as microcomputer and programmable controllers.

The problem can be mitigated by using filters, isolation transformers, and some line conditioners.

Others

Notching

PQVoltage

FluctuationFrequencyVariation

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Voltage Unbalance

• Deviation of magnitude and/or phase in 3-phase system

• Can result in heating of induction motors

Notching– Caused by commutation in semiconductor

converters

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What is a Transient or Surge?• Transients, commonly called as surges are sub

cycle disturbances of very short duration that vary greatly in magnitude.

• When transient occurs, thousands of voltage can be generated into the electrical system

• A Transient can be classified into two categories, impulsive and oscillatory Duration < 50 ns to 50 ms

• .00000005 seconds to .002 seconds• .000005 seconds to .050 secondsApril 19, 2023 59

Oscillatory & Impulsive Transient

Oscillatory Transient is a sudden, non – power frequency change, include both positive & negative polarity values.

Impulsive transient is a sudden, non – power frequency change, unidirectional in polarity ( primarily either positive or negative)

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Sources of Transients

Lightning• Static• Arc Welding

Switching• Contactor• Relays• SCR’s

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Harmonic Indices

• Two important indices, THD and TDD, are used to describe the effects of harmonics on power system components and communication systems.

• These indices are used to measure the deviation of a periodic waveform containing harmonics from a perfect sine wave.

• For a perfect sine wave, the deviation (or the distortion) is zero.

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IEEE 519-1992

IEEE standard 519 - 1992 specifies the allowable limits for voltage and current distortion at various bus and system voltages. These are given in Tables 1 and 2.

The important terminology used in this standard are explained below:

• The Point of Common Coupling (PCC) is the location of the harmonic voltage and current distortion to be calculated or measured.

• Total Harmonic Distortion (THD) is the total harmonic voltage distortion calculated or measured at PCC.

• Total Demand Distortion (TDD) is the percentage of total harmonic current distortion calculated or measured at PCC.

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Total Demand Distortion (TDD)

The ratio of the root-mean- square of the harmonic current to the root-mean-square value of the rated or maximum demand fundamental current, expressed as a percent.

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Total Harmonic distortion (THD)

Total harmonic distortion (THD) is a ratio of harmonic distortion to the fundamental frequency.

The greater the THD the more distortion there is of the 60 Hz sine wave.

Harmonic distortion occurs in voltage and current waveforms. Typically, voltage THD should not exceed 5% and current THD should not exceed 20%. Some of the power meters offered by Siemens are capable of reading THD.

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100xlfundamentaofamplitudeofSquare

harmonicsallofamplitudesofsquaresofSumTHD

h

h

h

V

V

22

1

2

h

h

h

I

I

22

1

2

THD of a current wave form can be defined as,

x 100

X 100

THDV =

THDI =

Mathematically, THD of a voltage wave form can be defined as,

Total Harmonic distortion (THD)

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CBEMA: Computer Business Equipment Manufacturers Association

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