ELECTRIC POWER QUALITY

[open elective-2]

Presented by:

Sri. L.Vamsi Narasimha Rao M.Tech, (Ph.D.) , Assistant Professor,

Department of EEE, Sir C R Reddy College of Engineering.

 Syllabus-ELECTRIC POWER QUALITY
Unit–I: Introduction
Overview of power quality – Concern about the power quality – General classes of power quality and voltage quality problems – Transients – Long–duration
voltage variations – Short–duration voltage variations – Voltage unbalance – Waveform distortion – Voltage
fluctuation – Power frequency variations.

Unit–II: Voltage imperfections in power systems

Power quality terms – Voltage sags – Voltage swells and interruptions – Sources of voltage sag, swell and interruptions – Nonlinear loads – IEEE and IEC
standards. Source of transient over voltages – Principles of over voltage protection – Devices for over voltage protection –
Utility capacitor switching transients.

Unit–III: Voltage Regulation and power factor improvement:

Principles of regulating the voltage – Device for voltage regulation – Utility voltage regulator application – Capacitor for voltage regulation – End–user
capacitor application – Regulating utility voltage with distributed resources – Flicker – Power factor penalty – Static VAR compensations for power factor
improvement.

Unit– IV: Harmonic distortion and solutions

Voltage distortion vs. Current distortion – Harmonics vs. Transients – Harmonic indices – Sources of harmonics – Effect of harmonic distortion – Impact of
capacitors, transformers, motors and meters – Point of common coupling – Passive and active filtering – Numerical problems.

Unit–V: Distributed Generation and Power Quality

Resurgence of distributed generation – DG technologies – Interface to the utility system – Power quality issues and operating conflicts – DG on low voltage
distribution networks.

Unit–VI :Monitoring and Instrumentation

Power quality monitoring and considerations – Historical perspective of PQ measuring instruments – PQ measurement equipment – Assessment of PQ
measuring data – Application of intelligent systems – PQ monitoring standards.
Text Books
1.Electrical Power Systems Quality, Dugan R C, Mc Granaghan M F, Santoso S, and Beaty H
W, Second Edition, McGraw–Hill, 2012, 3rd edition.

2.Electric power quality problems –M.H.J.Bollen IEEE series-Wiley India publications,2011.

Reference Books:
1. Power Quality Primer, Kennedy B W, First Edition, McGraw–Hill, 2000.
2. Understanding Power Quality Problems: Voltage Sags and Interruptions, Bollen M
HJ, First Edition, IEEE Press; 2000.
3. Power System Harmonics, Arrillaga J and Watson N R, Second Edition, John Wiley
& Sons, 2003.
4. Electric Power Quality control Techniques, W. E. Kazibwe and M. H. Sendaula, Van
Nostrad Reinhold, New York.
5. Power Quality c.shankaran, CRC Press, 2001
6. Harmonics and Power Systems –Franciso C.DE LA Rosa–CRC Press (Taylor &
Francis)
7. Power Quality in Power systems and Electrical Machines–EwaldF.fuchs, Mohammad
A.S. Masoum–Elsevier.
Course Outcomes
At the end of this course the student should be able to

• Differentiate between different types of power quality problems.

• Explain the sources of voltage sag, voltage swell, interruptions, transients, long duration over

voltages and harmonics in a power system.

• Analyze power quality terms and power quality standards.

• Explain the principle of voltage regulation and power factor improvement methods.

• Demonstrate the relationship between distributed generation and power quality.

• Explain the power quality monitoring concepts and the usage of measuring instruments.
 Power quality can be tackled from three trends namely

1.Utility must design operate the power system using power quality problems.

2.End user must employ proper wiring and grounding.

3.Manufacturers must design electronic devices that keep electronic disturbances to a minimum

value.
The Power Quality Evaluation Procedure
General Classes of Power Quality Problems
• Given by IEEE, IEC
• Principal Phenomena Causing Electromagnetic Disturbances as Classified by the IEC.
• low-frequency phenomena
• Radiated low-frequency phenomena
• Conducted high-frequency phenomena
• For steady-state phenomena by IEC (International Electrotechnical Commission)
• Amplitude
• Frequency
• Spectrum
• Modulation
• Source impedance
• Notch depth
• Notch area
• For non-steady-state phenomena
• Rate of rise
• Amplitude
• Duration
• Spectrum
• Frequency
• Rate of occurrence
• Energy potential
• Source impedance
General Classes of Power Quality Problems Continuation……
• Categories and Characteristics of Power System Electromagnetic Phenomena
Transients

• transient is “that part of the change in a variable that disappears during

transition from one steady state operating condition to another steady
state operating condition
• Impulsive transient Oscillatory transient
 Transients
• transient is “that part of the change in a variable that disappears during
transition from one steady state operating condition to another
• Impulsive transient An impulsive transient is a sudden, non–power frequency
change in the steady-state condition of voltage, current, or
both that is unidirectional in polarity (primarily either positive
or negative).

Impulsive transients are normally characterized by their rise and

decay times, which can also be revealed by their spectral content

The most common cause of impulsive transients is lightning

Because of the high frequencies involved, the shape of impulsive

transients can be changed quickly by circuit components and may
have significantly different characteristics when viewed from
different parts of the power system
Figure .Lightning stroke current impulsive transient
 Oscillatory transient

An oscillatory transient is a sudden, non–power frequency

change in the steady-state condition of voltage, current, or
both, that includes both positive and negative polarity
values.
Classified in to high, medium, and low frequency

Figure .Oscillatory transient current caused by back-to-back

capacitor switching.
 Oscillatory transient Continued

Figure. Low-frequency oscillatory transient caused by

Figure. Low-frequency oscillatory transient caused
ferroresonance of an unloaded transformer.
by capacitor bank energization. 34.5-kV bus voltage.
Long-Duration Voltage Variations

• Long-duration variations encompass root-mean-square (rms)

deviations at power frequencies for longer than 1 min
• Long-duration variations can be either overvoltages or undervoltages.
• Over voltages and under voltages generally are not the result of
system faults, but are caused by load variations on the system and
system switching operations
Overvoltage Under voltage
An overvoltage is an increase in the RMS ac An under voltage is a decrease in the RMS ac voltage
voltage greater than 110 percent at the power to less than 90 percent at the power frequency for a
frequency for a duration longer than 1 min. duration longer than 1 min.

Over voltages are usually the result of load Under voltages are the result of switching events that
switching are the opposite of the events that cause over voltages

Incorrect tap settings on transformers can also Overloaded circuits can result in under voltages
result in system over voltages.
 Short-Duration Voltage Variations
• This category encompasses the IEC category by
• Short-duration voltage variations are caused by fault conditions, the
energization of large loads which require high starting currents, or
intermittent loose connections in power wiring.
Short-Duration Voltage Variations
• Interruption
An interruption occurs when the supply voltage or load current
decreases to less than 0.1 pu for a period of time not exceeding 1 min.
• Interruptions can be the result of power system faults, equipment failures, and
control malfunctions. The interruptions are measured by their duration since the
voltage magnitude is always less than 10 percent of nominal.

Figure. Three-phase rms voltages for a

momentary interruption due to a fault and
subsequent recloser operation.
 Short-Duration Voltage Variations
• Sags (dips)
A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at
the power frequency for durations from 0.5 cycle to 1 min.
• Voltage sags are usually associated with system faults but can also be
caused by energization of heavy loads or starting of large motors.

Figure. Voltage sag caused by an SLG fault. (a) RMS waveform for voltage sag event. (b) Voltage sag waveform.
Short-Duration Voltage Variations
• Sags (dips)

Figure. Temporary voltage sag caused by motor starting.

Short-Duration Voltage Variations
• Swells
A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or
current at the power frequency for durations from 0.5 cycle to 1 min.
• As with sags, swells are usually associated with system fault conditions, but they
are not as common as voltage sags.
• Swells can also be caused by switching off a large load or energizing a large
capacitor bank.

Figure. Instantaneous voltage swell caused by

an SLG fault.
Voltage Imbalance
• Voltage imbalance (also called voltage unbalance) is defined as the
maximum deviation from the average of the three-phase voltages or
currents, divided by the average of the three-phase voltages or currents,
expressed in percent.
The primary source of voltage
unbalances of less than 2 percent is
single-phase loads on a three-phase
circuit. Voltage unbalance can also
be the result of blown fuses in one
phase of a three-phase capacitor
bank.

Figure. Voltage unbalance trend for a residential feeder.

 Waveform Distortion
Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power
frequency principally characterized by the spectral content of the deviation.

DC offset.
The presence of a dc voltage or current in an ac power system is termed dc offset.
This can occur as the result of a geomagnetic disturbance or asymmetry of electronic power converters.
Harmonics.
Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at
which the supply system is designed to operate
 Waveform Distortion
Harmonics.
Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at
which the supply system is designed to operate.
The Total harmonic distortion)THD is the measure of the effective valve of the harmonic components of a
distorted waveform

Figure. Distorted sine wave

Harmonics
 Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at
which the supply system is designed to operate.
 Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system.
 Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase
angles of each individual harmonic component.
 It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the effective
value of harmonic distortion.
 The waveform and harmonic spectrum for a typical adjustable-speed-drive (ASD) input current. Current distortion
levels can be characterized by a THD value, as previously described, but this can often be misleading.
 Waveform Distortion
Interharmonics:

 Voltages or currents having frequency components that are not integer multiples of the frequency at which the
supply system is designed to operate (e.g., 50 or 60 Hz) are called interharmonics.

 They can appear as discrete frequencies or as a wideband spectrum.

 Interharmonics can be found in networks of all voltage classes. The main sources of interharmonic waveform
distortion are static frequency converters, cycloconverters, induction furnaces, and arcing devices.

 Power line carrier signals can also be considered as interharmonics

 Waveform Distortion
Notching
 Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices when
current is commutated from one phase to another.
 The notches occur when the current commutates from one phase to another.
 During this period, there is a momentary short circuit between two phases, pulling the voltage as close to zero as
permitted by system impedances.

Figure. Example of voltage notching

caused by a three-phase converter.
 Waveform Distortion
Noise:

 Noise is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed
upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines.

 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 often exacerbated by improper grounding that fails to conduct noise away from the power
system.

 Basically, noise consists of any unwanted distortion of the power signal that cannot be classified as harmonic
distortion or transients.

 Noise disturbs electronic devices such as microcomputer and programmable controllers.

 The problem can be mitigated by using filters, isolation transformers, and line conditioners.
Voltage Fluctuation:
 Voltage fluctuations are systematic variations of the voltage envelope or a series of random voltage changes,
the magnitude of which does not normally exceed the voltage ranges.
 Loads that can exhibit continuous, rapid variations in the load current magnitude can cause voltage variations
that are often referred to as flicker.
 The term flicker is derived from the impact of the voltage fluctuation on lamps such that they are perceived
by the human eye to flicker.
 To be technically correct, voltage fluctuation is an electromagnetic phenomenon while flicker is an
undesirable result of the voltage fluctuations in some loades.

Fig: Example of voltage fluctuations caused by arc furnace operation

 Voltage Fluctuation
• voltage fluctuation is an electromagnetic phenomenon while flicker is an undesirable result of the voltage
fluctuation in some loads.
• Voltage fluctuations are systematic variations of the voltage envelope or a series of random voltage changes,
the magnitude of which does not normally exceed the voltage ranges of 0.9 to 1.1 pu.
• Loads that can exhibit continuous, rapid variations in the load current magnitude can cause voltage variations
that are often referred to as flicker.
• Typically, magnitudes as low as 0.5 percent can result in perceptible lamp flicker if the frequencies are in the
range of 6 to 8 Hz.
• According to IEC 61000-4-15 Flickers are
• short-term flicker sensation (Pst) – voltage fluctuations sufficient to cause
noticeable flicker to 50 percent of a sample observing group.
• Pst samples are normally reported at 10-min intervals
• long-term flicker sensation (Plt) - This value is a longer-term average of Pst samples.
• The Plt value is produced every 2 hr from the Pst values.
 Power Frequency Variations:
 Power frequency variations are defined as the deviation of the power system fundamental frequency from it
specified nominal value (e.g., 50 or 60 Hz).

 The power system frequency is directly related to the rotational speed of the generators supplying the system.

 There are slight variations in frequency as the dynamic balance between load and generation changes.

 The size of the frequency shift and its duration depend on the load characteristics and the response of the
generation control system to load changes.

 Frequency variations that go outside of accepted limits for normal steady-state operation of the power system can
be caused by faults on the bulk power transmission system, a large block of load being disconnected, or a large
source of generation going off-line.
 On modern interconnected power systems, significant frequency variations are rare.
 Frequency variations of consequence are much more likely to occur for loads that are supplied by a generator
isolated from the utility system.
 In such cases, governor response to abrupt load changes may not be adequate to regulate within the narrow
bandwidth required by frequency-sensitive equipment.
 Voltage notching can sometimes be mistaken for frequency deviation.
 The notches may come sufficiently close to zero to cause errors in instruments and control systems that rely on
zero crossings to derive frequency or time.