POWER QUALITY AND

CUSTOM POWER

DEPARTMENT OF ELECTRICAL ENGINEERING

ODISHA UNIVERSITY OF TECHNOLOGY AND RESEARCH, ODISHA
POWER QUALITY
• Power quality is ultimately a consumer-driven issue.
• Accordingly the definition power quality may be given as
Any power problem manifested in voltage, current, or frequency
deviations that results in failure or misoperation of customer equipment.
• Power Quality =Voltage Quality
• The power supply system can only control the quality of the voltage;
it has no control over the currents that particular loads might draw.
• Therefore, the standards in the power quality area are devoted to
maintaining the supply voltage within certain limits.
Terms and Definitions

• We will discuss consistent terminology that can be used to describe power

quality variations.
General Classes of Power Quality Problems:
Institute for Electrical and Electronics Engineers (IEEE):
IEEE Standards Coordinating Committee 22 (IEEE SCC22)
International Electrotechnical Commission (IEC)
Congress Internationale des Grand Réseaux Électriques a
Haute Tension
(CIGRE; in English, International Conference on Large
High-Voltage Electric Systems)
Phenomena Causing Electromagnetic
Disturbances
• Electromagnetic Disturbances as Classified by the IEC
1. Conducted low-frequency phenomena
➢Harmonics, interharmonics
➢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
Electromagnetic Disturbances as Classified by the IEC
2. Radiated low-frequency phenomena
➢Magnetic fields
➢Electric fields
3.Conducted high-frequency phenomena
➢Induced continuous-wave (CW) voltages or currents
➢Unidirectional transients
➢Oscillatory transients
4. Radiated high-frequency phenomena
➢Magnetic fields
➢Electric fields
➢Electromagnetic fields
➢Continuous waves
➢Transients
Categorization of Electromagnetic Phenomena

Attributes used for steady-state phenomena

• Amplitude
• Frequency
• Spectrum
• Modulation
• Source impedance
• Notch depth
• Notch area
Categorization of Electromagnetic Phenomena

Attributes used for non-steady-state phenomena:

• Rate of rise
• Amplitude
• Duration
• Spectrum
• Frequency
• Rate of occurrence
• Energy potential
• Source impedance
Categories and Characteristics of Power System Electromagnetic
Phenomena
• Transients
• Short-duration variations
• Long-duration variations
• Voltage unbalance
• Waveform distortion
• Voltage fluctuations
• Power frequency variations
Transients

• Transients are short-duration events, the characteristics of which

are predominantly determined by the resistance, inductance, and
capacitance of the power system network at the point of interest.
It is an event that is undesirable and momentary in nature
OR
That part of the change in a variable that disappears during
transition from one steady state operating condition to another.
Transients

Impulsive Oscillatory
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).

• Example: A 1.2 × 50-µs 2000-volt (V) impulsive transient

• The most common cause of impulsive transients is lightning

 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.
• In other words, the instantaneous voltage or current value of an
oscillatory transient varies its polarity quickly. It is described by
its spectral content or predominant frequency, magnitude and
duration.
• Frequency of the oscillation gives a trace to the origin of the
disturbance.
Oscillatory transient

Voltage

Time in ms
Oscillatory Transient Due to Back-to-Back Capacitor Switching
Oscillatory transient

Capacitor Switching Transients

 Long-Duration Voltage Variations
• Long-duration variations encompass root-mean-square (rms)
deviations at power frequencies for longer than 1 min.
• A voltage variation is considered to be long duration when the
ANSI limits are exceeded for greater than 1 min.
• ANSI C84.1 specifies the steady-state voltage tolerances expected
on a power system
• Long-duration variations can be either over-voltages or under-
voltages.
• 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

• An overvoltage is an increase in the rms ac voltage greater than 110

percent at the power frequency for a duration longer than 1 min.
Cause of Over Voltage
• Switching off a large load
• Energizing a capacitor bank
• Incorrect tap settings on transformers
 Undervoltage

• 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.
• Under voltages are the result of:
• Switching events that are the opposite of the events that cause
over voltages.
• Switching of Large Loads
• Switching off a capacitor bank
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.
• The term sustained interruption refers to specific power system
phenomena.
• This term has been defined to be more specific regarding the
absence of voltage for long periods.
Short-Duration Voltage Variations

• This category belongs to the IEC category of voltage dips and

short interruptions. Each type of variation can be designated as
instantaneous, momentary, or temporary, depending on its
duration.
Caused By
• fault conditions
• energization of large loads, which require high starting currents
• intermittent loose connections in power wiring
Short-Duration Voltage Variations

Depending on the fault location and the system conditions

• voltage drops (sags)
• voltage rises (swells) or
• a complete loss of voltage (interruptions).

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
Interruption

Three-phase rms
voltages for a
momentary
interruption due to
a fault and
subsequent
recloser operation
 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.
• The IEC definition for this phenomenon is dip.

Voltage sags associated with system faults or

• energization of heavy loads or
• starting of large motors
Sag

Voltage sag caused by an SLG fault. (1) RMS waveform for voltage
sag event. (2) Voltage sag waveform
 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.
• One way that a swell can occur is from the temporary voltage rise
on the unfaulted phases during an SLG fault
• Swells can also be caused by switching off a large load or
energizing a large capacitor bank
Swell
Voltage Imbalance
• Voltage imbalance (also called voltage unbalance) is sometimes
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.
• Severe voltage unbalance (greater than 5 percent) can result from
single-phasing conditions.
 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.
There are five primary types of waveform distortion:
• DC offset
• Harmonics
• Interharmonics
• Notching
• Noise
Waveform Distortion
Power Frequency Variations
• Power frequency variations are defined as the deviation of the
power system fundamental frequency from it specified nominal
value.

frequency variations for a 24-h period on a typical 13-kV substation bus

Power Quality Terms
• Active filter: Any of a number of sophisticated power electronic
devices for eliminating harmonic distortion. See passive filter.
• CBEMA curve: A set of curves representing the withstand
capabilities of computers in terms of the magnitude and duration
of the voltage disturbance.
(Developed by the Computer Business Equipment Manufacturers
Association) (CBEMA)
CBEMA has been replaced by the Information Technology Industry
Council (ITI), and a new curve has been developed that is commonly
referred to as the ITI curve. See ITI curve.
Common Mode Voltage: The noise voltage that appears equally
from current carrying conductor to ground.
Power Quality Terms
• Coupling: A circuit element, or elements, or a network that
may be considered common to the input mesh and the output
mesh and through which energy may be transferred from one to
another
• Crest factor: A value reported by many power quality
monitoring instruments representing the ratio of the crest value
of the measured waveform to the root mean square of the
fundamental. For example, the crest factor of a sinusoidal wave
is 1.414.
• critical load: Devices and equipment whose failure to operate
satisfactorily jeopardizes the health or safety of personnel,
and/or results in loss of function, financial loss, or damage to
property deemed critical by the user.
Power Quality Terms
• Current distortion: Distortion in the ac line current
• Differential mode voltage: The voltage between any two of a
specified set of active conductors
• Dip:
• Distortion: Any deviation from the normal sine wave for an ac
quantity.
• Distributed generation (DG): Generation dispersed throughout
the power system as opposed to large, central station power
plants. In the context used in this book, DG typically refers to
units less than 10 megawatts (MW) in size that are
interconnected with the distribution system rather than the
transmission system.
 Power Quality Terms

• Dropout: A loss of equipment operation (discrete data signals) due to

noise, sag, or interruption.
• Dropout voltage: The voltage at which a device will release to its
deenergized position (for this document, the voltage at which a device
fails to operate).
• Electromagnetic compatibility: The ability of a device, equipment, or
system to function satisfactorily in its electromagnetic environment
without introducing intolerable electromagnetic disturbances to anything
in that environment.
Power Quality Terms
• equipment grounding conductor: The conductor used to connect
the non–current carrying parts of conduits, raceways, and equipment
enclosures to the grounded conductor (neutral) and the grounding
electrode at the service equipment (main panel) or secondary of a
separately derived system (e.g., isolation transformer).
• Given in National Fire Protection Association (NFPA) 70-1993,
Section 100

• Failure mode: The effect by which failure is observed.

 Power Quality Terms

• Fast tripping: Refers to the common utility protective relaying

practice in which the circuit breaker or line recloser operates
faster than a fuse can blow. Also called fuse saving. Effective
for clearing transient faults without a sustained interruption, but
is somewhat controversial because industrial loads are
subjected to a momentary or temporary interruption.
• Fault: Generally refers to a short circuit on the power system.
Voltage sag analysis and mitigation

• Voltage sags and interruptions are related power

quality problems.
Both are usually the result of:
• faults in the power system
• switching actions to isolate the faulted sections
Causes of Voltage Sags or Dips
• Electrical Faults
 Voltage sag analysis and mitigation
• Switching Large Loads in one instance
• Direct online starting of Large Rating Motors
Voltage sag analysis and mitigation
• Energizing large rating power transformer
• Arcing Fault in the electrical System.
• Tree or other objects falling on transmission line

Voltage Waveform during voltage sag

Sag Mitigation
• Objective is to reduce the number and severity of voltage sags
and to reduce the sensitivity of equipment to voltage sags.
Sag Mitigation
These Approach of mitigation are divided into three parts that
involves utility, customer and equipment manufacturer.
• Dynamic voltage restorer
• Active series Compensators
• Distribution static compensator (DSTATCOM)
• Solid state transfer switch (SSTS)
• Static UPS with energy storage
• Backup storage energy supply (BSES)
• Ferro resonant transformer
• Flywheel and Motor Generator set
• Static Var Compensator (SVC)
 Dynamic Voltage Restorer: (DVR)

• Dynamic Voltage Restorers (DVR) are complicated static devices

which work by adding the ‘missing’ voltage during a voltage sag.
Basically this means that the device injects voltage into the
system in order to bring the voltage back up to the level required
by the load.
 Active series Compensators
• It is a device that can boost the voltage by injecting a voltage in
series with the remaining voltage during a voltage sag condition.
These are referred to as active series compensation devices.
Schematics for Active series Compensators
Distribution static compensator (DSTATCOM)
• A DSTATCOM is a voltage source converter based
compensating device which is connected in parallel with the
distribution system to control the flow of reactive power.

The aim of the DSTATCOM

is to provide voltage
regulation at the load
point and mitigate the
voltage sag generated
when the load is increased
Solid state transfer switch (SSTS)
• In case of a power supply failure, the Solid-state Transfer Switch (SSTS) can
switch to the normal power supply with fast switching (5 milliseconds), to
prevent the sensitive loads from undervoltage or interruption effects.
Uninterrupted Power Supply (UPS)

• Uninterruptible power supplies (UPS) mitigate voltage sags by

supplying the load using stored energy. Upon detection of
voltage sag, the load is transferred from the mains supply to the
UPS.
• UPS systems have the advantage that they can mitigate all
voltage sags including outages for significant periods of time
(depending on the size of the UPS).
• There are 2 topologies of UPS available; on-line and off-line.
Figure shows a schematic of an off-line UPS and a schematic of
an on-line UPS.
Uninterrupted Power Supply (UPS)

Block Diagram of Offline UPS Block Diagram of Online UPS

Backup storage energy supply (BSES)
 Ferro resonant transformer
• A Ferro resonant transformer, also known as a constant voltage
transformer (CVT), is a transformer that operates in the saturation
region of the transformer B-H curve.
• Voltage sags down to 30% retained voltage can be mitigated
through the use of Ferro resonant transformers.
• Ferro resonant transformers are simple and relatively maintenance
free devices which can be very effective for small loads. Ferro
resonant transformers are available in sizes up to around 25 KVA.
• On the down side, the transformer introduces extra losses into the
circuit and is highly inefficient when lightly loaded. In some cases
they may also introduce distorted voltages
• In addition, unless greatly oversized, Ferro resonant transformers
are generally not suitable for loads with high inrush currents such
as direct-on-line motors.
Ferro resonant transformer

Schematic of a Ferro Resonant Transformer

Ferro resonant transformer
• If properly sized, a CVT can regulate its output voltage during a
voltage sag to 60% for any duration. However, CVTs are not effective
during momentary voltage interruptions or extremely deep voltage
sags (generally below fifty percent of nominal). CVTs are often
favored over other sag-mitigation devices because they are relatively
maintenance-free, with no batteries to replace or moving parts to
maintain.
• As mentioned, CVTs also have the special characteristic of being able
to store energy for up to 1/2 cycle because of its "tank circuit"
design. Combining CVT with an inverter and static transfer switch in
a UPS application, this characteristic provides a ride-through
capability allowing a completely uninterrupted transfer to an alternate
source. This application is very useful in the event of an overload,
fault or even in the event of a total loss of the inverter, thereby
maintaining power to the load.
 Flywheel and Motor Generator set
• A motor powered by the line drives a generator that powers the
load. Flywheels on the same shaft provide greater inertia to
increase ride-through time.

• When the line suffers a disturbance, the inertia of the machines

and the flywheels maintains the power supply for several seconds.

• This arrangement may also be used to separate sensitive loads

from other classes of disturbances such as harmonic distortion
and switching transients.
Flywheel and Motor Generator set

Flywheel Motor Generator Set

 Static Var Compensator (SVC)

• A SVC is a shunt connected power electronics based device

which works by injecting reactive current into the load, thereby
supporting the voltage and mitigating the voltage sag.
• SVCs may or may not include energy storage, with those systems
which include storage being capable of mitigating deeper and
longer voltage sags
CBEMA Curve

• CBEMA Curve represents typical equipment sensitivity

characteristics.
• Subsequently renamed the Information Technology
Industry Council (ITI)
• Typical loads will likely trip off when the voltage is below
the CBEMA, or ITI, curve
Voltage sag ride-through capability curves

Typical equipment voltage sag ride-through capability curves

 Sag magnitude and duration calculations
• The magnitude of a voltage sag is determined from the rms
voltage
 Sag magnitude and duration calculations
• The rms voltage has been calculated over a one-cycle sliding window
as:

Voltage Sag Magnitude: Calculation

The voltage at the pcc is found as:

With condition that E=1 p.u

Voltage divider model for a voltage sag
Equipment performance in presence of sag
Voltage sags pose a serious power quality issue for the
electric power industry
Reference: Power Systems Engineering Research Center
Report, PSERC Publication 05-63, October 2005
• Contactors: Test results for 10 and 15 ampere contactors
showed that the contactors were not affected by sags to depths
of 70%. Chattering occurred for sag depths of 60% and duration
greater than 30 cycles. In the case of 50% and 40% sag depths,
the contactors tripped without chattering for all sag durations.
The contactors behaved identically under load and no-load
conditions. Circuit breakers were unaffected.
• Lamps. Reduced light intensity occurred for the tested helium
and fluorescent lamps. The reduction depended on sag depth,
but not duration.
 Equipment performance in presence of sag
• Air Conditioners: The compressor motors stalled for sags greater
than 50% and durations greater than 10 cycles. The point of initiation
of the voltage sag in the voltage cycle did not noticeably affect motor
performance. However, as expected, motor speed decreased and
current increased during the sags.
• Computers: Computers restarted for sags of depth greater than
30% and duration longer than 8 cycles.
• Microwave Ovens: Microwave ovens switched off for 50% sag
depth and duration of 10 cycles or more. They also switched off at
60% sag depth and duration of 30 cycles or more. There were only
visible effects (such as flickering of light inside the oven and blinking
of a digital clock) for sags of depth 90%, 80%, and 70%.
 Equipment performance in presence of sag
• Televisions: The televisions switched off for 50% sag depth and
duration of 30 cycles or more. The switching off was preceded by
shrinking and collapsing of the video image.
• Audio and Video Equipment: Performance of DVD/ VHS players
and stereo CD players was largely unaffected by sags, except for
flickering of the electronic timer displays.
• Digital Alarm Clock Radios: Digital alarm clock radios suffered
severe audio quality loss for sags of depths 60% and 50% for a few
seconds.
• Toasters: Sags of 60% depth and 50 cycle duration, and 50% depth
and 40 cycle duration caused a toaster that had just begun operation
to turn off automatically because the coils of the toaster did not get
red hot due to the sag. However, the sag had no effect on already
operating toasters.
Effects of Voltage Sags on Adjustable Speed Drives

• Adjustable speed drives (ASD) are very susceptible to slight

variation in voltages. The reason for their high susceptibility is the
presence of power electronics components that are sensitive to
voltage variation.
The various factors determining the performance of AC motor
drives during voltage sags are:
• Sag magnitude variation
• Sag duration
• Sag asymmetry
• Phase jump
• Non-sinusoidal wave shapes
Effects of Voltage Sags on Adjustable Speed Drives
The main reasons for AC drive tripping during voltage sag
are:
1. DC link under voltage
2. Drop in speed of motor load
3. Increased AC currents during sag or post-sag over currents
charging the DC capacitor.
Schematics of AC and DC Drives

A DC adjustable speed drive

An AC adjustable speed drive

Module-II
HARMONICS
Harmonics are described by IEEE as sinusoidal voltages or
currents having frequencies that are integer multiples of the
fundamental frequency at which the power system is designed to
operate.
➢Harmonics combine with the fundamental voltage or current
producing a non-sinusoidal shape, thus, a waveform distortion
power quality problem
HARMONICS

Harmonic distortion is caused by nonlinear devices in the

power system.
What is a nonlinear device?
• A nonlinear device is one in which the current is not proportional
to the applied voltage.
HARMONICS
• Any periodic, distorted waveform can be expressed as a sum of
sinusoids.

• Harmonic distortion levels can be characterized by the complete

harmonic spectrum with magnitudes and phase angles of each
individual harmonic component
HARMONICS
• Sine-wave voltage generated in central power stations is very
good.
• In most areas, the voltage found on transmission systems typically
has much less than 1.0 percent distortion.
• However, the distortion increases closer to the load. At some
loads, the current waveform barely resembles a sine wave.
Voltage versus Current Distortion
 HARMONICS
• Voltage harmonics appearing at the load bus.
• The amount of voltage distortion depends on the impedance and
the current.
• While the load current harmonics ultimately cause the voltage
distortion. (IEEE Standard 519-1992)
Therefore, Harmonics always originate as current harmonics
and voltage harmonics are the results of current harmonics.
• Current harmonics originate because of the presence of non-linear
loads.
HARMONICS
HARMONICS
• IEEE 519 specifies the harmonic limits on Total Demand
Distortion (TDD)
• TDD represents the amount of harmonics with respect to the
maximum load current over a considerable period of time (not the
maximum demand current)
Total Harmonic Distortion (THD)
• THD represents the harmonic content with respect to the actual
load current at the time of measurement.
Individual harmonic distortion (IHD)
• It is the ratio between the root mean square (RMS) value of the
individual harmonic and the RMS value of the fundamental.
HARMONIC ANALYSIS
Individual Harmonic Distortion
• Individual harmonic distortion (IHD) is the ratio between the root
mean square (RMS) value of the individual harmonic and the
RMS value of the fundamental.
IHDn = In/I1
Total Harmonic Distortion:
• Total harmonic distortion (THD) is a term used to describe the
net deviation of a nonlinear waveform from ideal sine waveform
characteristics.
• Total harmonic distortion is the ratio between the RMS value of
the harmonics and the RMS value of the fundamental.
THD = (IH/I1) × 100%
HARMONIC ANALYSIS

• 𝐼𝐻 = √(𝐼22 + 𝐼32 + 𝐼42 + 𝐼52 + ⋯

• The individual harmonic distortion indicates the contribution of
each harmonic frequency to the distorted waveform, and the
total harmonic distortion describes the net deviation due to all
the harmonics.
Harmonic Sources from Commercial Loads
• Single-phase power supplies
• Fluorescent lighting
• Adjustable-speed drives for HVAC and elevators
Harmonic Sources from Industrial Loads
• Three-phase power converters
• Arcing devices
• Saturable devices
Harmonic Sources from Industrial Loads
Three-phase Power Converters
• Three-phase electronic power converters differ from single-phase
converters mainly because they do not generate third-harmonic currents.
• However, they can still be significant sources of harmonics at their
characteristic frequencies.
Arcing Devices
• This category includes arc furnaces, arc welders, and discharge-
type lighting (fluorescent, sodium vapor, mercury vapor) with
magnetic ballasts.
• The arc is basically a voltage clamp in series with a reactance
that limits current to a reasonable value.

Equivalent circuit for an arcing device.

Saturable Devices
• Equipment in this category includes transformers and other
electromagnetic devices with a steel core, including motors.
Harmonics are generated due to the nonlinear magnetizing
characteristics of the steel.
• Transformers under no load and light loads
Effects of Harmonic Distortion
• Harmonic currents produced by nonlinear loads are injected back
into the supply system.
• These currents can interact adversely with a wide range of power
system equipment, most notably capacitors, transformers, and
motors, causing additional losses, overheating.
• These harmonic currents can also cause interference with
telecommunication lines and errors in power metering.
Impact on Capacitors
Effects of Harmonic Distortion
Impact on Transformers
• Transformers are designed to deliver the required power to the
connected loads with minimum losses at fundamental frequency.
Harmonic distortion of the current, in particular, as well as of the
voltage will contribute significantly to additional heating
Impact on Motors
• Motors can be significantly impacted by the harmonic voltage distortion.
• Harmonic voltage distortion at the motor terminals is translated into
harmonic fluxes within the motor.
• Harmonic fluxes do not contribute significantly to motor torque, but rotate
at a frequency different than the rotor synchronous frequency, basically
inducing high-frequency currents in the rotor.
• The additional fluxes do little more than induce additional losses.
Decreased efficiency along with heating, vibration, and high-pitched noises
are indicators of harmonic voltage distortion.
Effects of Harmonic Distortion

Impact on Telecommunications:
• Harmonic currents flowing on the utility distribution system or within
an end-user facility can create interference in communication
circuits sharing a common path
Effects of Harmonic Distortion
Impact on energy and demand metering
Electric utility companies usually measure energy consumption in
two quantities: the total cumulative energy consumed and the
maximum power used for a given period.
There are two charges in any given billing period especially for
larger industrial customers: 1) energy charges and 2) demand
charges.
Residential customers are typically charged for the energy
consumption only.
• Harmonic currents from nonlinear loads can impact the
accuracy of watthour and demand meters adversely.
• Conventional magnetic disk watthour meters tend to have a
negative error at harmonic frequencies.
Effects of Harmonic Distortion

• The meter register low for power at harmonic frequencies if they

are properly calibrated for fundamental frequency.
• This error increase.es with increasing frequency.
• In general, nonlinear loads tend to inject harmonic power back
onto the supply system and linear loads absorb harmonic power
due to the distortion in the voltage.
Effects of Harmonic Distortion

• Thus for the nonlinear load, the meter would read

P measured= P1- a3 P3- a5 P5- a7 P7…………………..
• Where a3, a5, and a7 are multiplying factors (<1.0) that
represent the inaccuracy of the meter at harmonic frequency.

• In the case of the linear load, the measured power is:

P measured= P1+ a3 P3+ a5 P5+ a7 P7…………………..
Interharmonics
• The non-integer multiple of the fundamental frequency is
commonly known as an interharmonic frequency.
• Interharmonic frequencies are frequencies between two
adjacent harmonic frequencies.
• Primary source of interharmonics is the widespread use of
electronic power converter loads capable of producing current
distortion over a whole range of frequencies
Block diagram of a modern induction furnace with a current source
inverter
 Power Quality Monitoring
• Power quality monitoring is the process of gathering, analyzing,
and interpreting raw measurement data into useful information.

The monitoring objectives:

➢ To determine the choice of monitoring equipment.
➢ triggering thresholds.
➢ methods for data acquisition and storage.
➢ and to analysis and interpretation requirements.
 Power Quality Monitoring

• Several common objectives:

➢ Monitoring to characterize system performance

➢ Monitoring to characterize specific problems

➢ Monitoring as part of an enhanced power quality service

➢ Monitoring as part of predictive or just-in-time maintenance

 Power Quality Monitoring
Objective of Power Quality Monitoring
• To Quantify power quality phenomena at a particular location of
electrical power equipment.
• To diagnose incompatibility between the supply and the consumer
loads.
• To predict the performance of the load equipment and select
power quality mitigation systems.
• To find out the choice of monitoring equipment, the method of
collecting data, etc at a particular location.
Power Quality Monitoring
• Continuous Power Quality Monitoring detects, records and
leads to the prevention of Power Quality Issues.
• Power Quality Monitoring provides a continuous ‘Health Check’
of a facility like Harmonics, Transients due to Load Switching
etc can be monitored.

Power Quality monitoring may be done:

• At the Utility
• At the Customers end
• Any other personal such as energy auditors
Power Quality Monitoring
Power Quality Monitors must detect and record:
• Transients
• Interruptions
• Sag/Under Voltage
• Swell/Over Voltage
• Waveform Distortion
• Voltage Fluctuations
• Frequency Variations
 Power Quality Monitoring
Few Facts behind the Power Quality Monitoring Requirements:
• To find out the need for mitigation of power quality problems
• To schedule preventive and predictive maintenance
• To ensure the performance of equipment
• To assess the sensitivity of equipment to power quality
disturbances
• To identify power quality events and problems
• To reduce the power losses in the process and distribution system
• To reduce the loss in production and to improve equipment
availability
Power quality measuring equipment
These equipment include to measure everything from very fast
transient over voltages (microsecond time frame) to long-duration
outages (hours or days time frame), steady-state phenomena, such
as harmonic distortion, and intermittent phenomena, such as
voltage flicker.
Basic categories of instruments that may be applicable include:

■ Multimeters
■ Oscilloscopes
■ Disturbance analyzers
■ Harmonic analyzers and spectrum analyzers
■ Combination disturbance and harmonic analyzers
■ Flicker meters
■ Energy monitors
Power quality measuring equipment
Important factors that should be considered when selecting the
instrument are:
• Number of channels (voltage and/or current)
• Temperature specifications of the instrument
• Ruggedness of the instrument
• Input voltage range (e.g., 0 to 600 V)
• Power requirements
• Ability to measure three-phase voltages
• Input isolation (isolation between input channels and from each input to ground)
• Ability to measure currents
• Housing of the instrument (portable, rack-mount, etc.)
• Ease of use (user interface, graphics capability, etc.)
• Documentation
• Communication capability (modem, network interface)
• Analysis software
Power quality measuring equipment
Power quality measuring equipment
FLICKER METER
• Flicker meter is an instrument designed to measure any quantity
representative of flicker. Flicker is an impression of unsteadiness of
visual sensation induced by a light stimulus whose luminance or
spectral distribution fluctuates with time.
• Its main function is to provide assessment of the flicker perception
caused by voltage fluctuations.
• Therefore, the flicker meter should be designed to have the
capability of transforming the input voltage fluctuations into an output
parameter proportionally related to flicker perception.
• A flicker meter is essentially a device that demodulates the flicker
signal, weights it according to established “flicker curves,” and
performs statistical analysis on the processed data.
FLICKER METER
These meters can be divided up into three sections.
• In the first section the input waveform is demodulated, thus removing
the carrier signal. As a result of the demodulator, a dc offset and
higher-frequency terms (sidebands) are produced.
• The second section removes these unwanted terms using filters, thus
leaving only the modulating (flicker) signal remaining. The second
section also consists of filters that weight the modulating signal
according to the particular meter specifications.
• The last section usually consists of a statistical analysis of the
measured flicker.
FLICKER METER
The most established method for Flicker Measurement is
described in IEC Standard 61000-4-15.8 The IEC flicker meter
consists of five blocks, which are shown below
FLICKER METER
 FLICKER METER
• Block 1 is an input voltage adapter that scales the input half-cycle
rms value to an internal reference level. This allows flicker
measurements to be made based upon a percent ratio rather than
be dependent upon the input carrier voltage level.
• Block 2 is simply a squaring demodulator that squares the input to
separate the voltage fluctuation (modulating signal) from the main
voltage signal (carrier signal), thus simulating the behavior of the
incandescent lamp. In other words, the objective of this block is to
recapture the modulating signals while at the same time suppress
the mains frequency carrier signal.
• Block 3 consists of multiple filters that serve to filter out unwanted
frequencies produced from the demodulator and also to weight the
input signal according to the incandescent lamp eye-brain
response.
FLICKER METER
a. Demodulator Filters
• First-order high-pass (cutoff frequency = 0.05 Hz)
• Sixth-order low-pass Butterworth (cutoff frequency = 35 Hz)
b. Weighting Filter
• Band-pass filter (models the frequency-selective behavior of the
human eye)
• The basic transfer function for the weighting filter is shown
below.
 FLICKER METER
• Block 4 consists of a squaring multiplier and sliding mean filter. The
voltage signal is squared to simulate the nonlinear eye-brain
response, while the sliding mean filter averages the signal to
simulate the short term storage effect of the brain. The output of this
block is considered to be the instantaneous flicker level. A level of 1
on the output of this block corresponds to perceptible flicker.
• Block 5 consists of a statistical analysis of the instantaneous flicker
level. The output of block 4 is divided into suitable classes, thus
creating a histogram. A probability density function is created based
upon each class, and from this a cumulative distribution function
can be formed.
Subsequently, a number of points from the cumulative distribution
function are selected to calculate the short-term flicker severity (P-
ST), as well as the long-term flicker severity (P-LT).
 FLICKER METER
Flicker Severity
Flicker level evaluation can be classified into short-term and long-
term flicker severity.
a. Short-term Flicker Severity (PST)
This is based upon an observation period of 10 minutes, allowing
evaluation of disturbances with a short duty cycle or those that
generate continuous fluctuations. PST can be calculated using the
equation shown:

where the percentages P0.1, P1s, P3s, P10s, P50s are the flicker levels
that are exceeded 0.1, 1.0, 3.0, 10.0, and 50.0 percent of the time.
These values are taken from the cumulative distribution function.
A PST of 1.0 unit on block 5 output represents irritable flicker.
 FLICKER METER
b. Long-term Flicker Severity (PLT)
On the other hand, the need for long-term assessment of flicker
severity happens if the duty cycle is long or variable. These include
electric arc furnaces or disturbances on the system that are caused
by multiple loads operating simultaneously.
PLT is derived from PST as shown below.

where N is the number of PST readings and is determined by the duty

cycle of the flicker-producing load.
FLICKER METER

• The purpose is to capture one duty cycle of the fluctuating load.

If the duty cycle is unknown, the recommended number of PST
readings is 12 (2-h measurement window/ two hours of
measuring).
• The number of PST readings (N) is determined by the duty cycle
of the flicker-producing load, in order to capture one duty cycle
of the fluctuating load. However, if the duty cycle is unknown,
the recommended number of PST readings is 12 (two hours of
measuring). The limit for PLT is 0.8 units
 FLICKER METER
• IEEE Standards 141-19936 and 519-19927 both contain flicker
curves that have been used as guides for utilities to evaluate the
severity of flicker within their system.
• Both flicker curves, from Standards 141 and 519, are shown below
FLICKER METER
• Flicker variations at the PCC with an arc furnace characterized by
the Pst levels for a 24-h period (March 1, 2001) (note that there is
one Pst value every 10 min).
FLICKER METER

Flicker of a light Generated by Arc Furnace

FLICKER METER

Change in luminous flux resulting from a temporary voltage change

Mitigation Techniques and Devices

Steps to mitigate Power Quality issues

A. During the Production of Equipment
B. Analysis of the Causes
C. Power Conditioning Equipment
Power Passive Filters
Mitigation Techniques and Devices

Passive filters:
• Shunt Passive Filters
• Series Passive Filters

Shunt Passive Filters

Passive Shunt Filters
Series Passive Filters

• Passive Series Filters are connected in Series with the Loads to

provide high impedance to the harmonic currents by blocking the
harmonic currents from the source.

Series Passive Filters

Hybrid Passive Filters
Hybrid Passive Filters with Different Topology
Power Filters
Application of Passive Power Filter

1. Passive power filter are used in high power rating such as

HVDC, due to its :
✓ Simplicity
✓ Low cost,
✓ Robust structure and
✓ Benefits of meeting reactive power requirements at fundamental
frequency in most of the applications.
2. Extensively used i.n hybrid configuration.
(Major portion of filtering is taken up by passive filters )
3. In medium and low power rating distribution system because
of their low cost and simplicity.
Problem With Passive Power Filters

• The passive filters are very sensitive to the parallel resonance

between the capacitors of passive filter and the source impedance.
(which is highly inductive in nature)
• If the parallel resonance frequency occurs at or near a harmonic
produced by the load, a severe voltage distortion and harmonic
current amplification may be produced.
• It may result in nuisance fuse blowing and or breaker operation.
Use of Passive Power Filters

• Therefore, utmost care must be taken in the design of passive

filters to avoid such as parallel resonance and associated
problems.
• However, if these passive filters are used along with a small
active filter which blocks or avoids such parallel resonance.
Use of Passive Shunt Filter

Shunt passive filter have been consider more appropriate

• To mitigate the harmonic currents.
• Partially to meet the reactive power requirement of load.
• To relieve ac network from this problem especially current fed
types of nonlinear loads.
Use of Passive Series Filter
• In voltage fed type of load passive series filter are considered
better for blocking the harmonic
 Passive Filters
• The basic principle of operation of passive power filters may be
explained through their objective, location, connections, quality,
sharpness, rating, size, cost, detuning, applications and other
factors.
• The main purpose of passive shunt filters is to reduce the
harmonics voltage and current in the ac power system to an
acceptable level. And the basic operating principle of passive shunt
harmonic filter is to absorb harmonic current in low impedance path
using a tuned series LC circuit.
• In case of passive series filter, it is to block the harmonic currents
entering into ac network by passive tuned LC circuit offering a high
impedance for harmonic currents.
Passive Filters
Objectives
• In case of passive series filter, it is to block the harmonic
currents entering into ac network by passive tuned LC circuit
offering a high impedance for harmonic currents.
(the filter should not draw any fundamental current from the supply as
they are put in shunt.)
• Passive shunt filters are connected in parallel to the load and
rated for the system voltage at PCC
• Passive Filters are the engineering solution for harmonic
reduction within an acceptable limit and not the elimination of
harmonics
Connection and Configuration
• These passive filters are used in shunt, series and hybrid
configuration.
Shunt Filter
• The passive shunt filter are tuned at a slightly lower frequency
at which they absorb the harmonics.
• For multiple harmonics, multiple tuned shunt RLC filters are
used to absorb these harmonics current.
• Voltage harmonics are reduced at PCC due to reduced
harmonics current flowing in to the ac network and less
harmonic voltage at PCC resulting in reduction in harmonic
voltages.
Connection and Configuration
• These passive shunt filters provide leading reactive power at
fundamental frequency, which is an additional requirement for
large rating ac-dc converters having in ac system

Connection and Configuration for Series Filter

• The passive series filters are tuned at slightly low frequency at
which they have to block harmonic current
• For mitigation of multiple harmonics multiple parallel tuned RLC
circuits are used to block these harmonic current.
• For reducing the number of series connected parallel tune LC
circuit of passive element a high block filter is used in series with
other branches to block all higher order harmonics current and
they are connected all in series
Connection and Configuration for Series Filter

• Passive shunt filters having a lagging reactive power at

fundamental frequency which is undesired features and it has
inductive voltage drop at fundamental frequency resulting in
poor voltage regulation across the load.
• In addition this passive filter have to be rated for full current and
their protection becomes an additional requirement. Therefore,
these passive series filters are not very popular in practice
Sharpness of Tuning of Passive Filter

• It is quantified in terms a quality factor of the inductor (Q) and

known as sharpness of tuning of the passive filter.
• Passive shunt filter it considered to be tuned corresponding to a
frequency at which the inductive reactance is equal to
capacitive reactance.
• Passive shunt filter tuned at lower frequencies are sharply
tuned and have higher value of quality factor typically order of
10 to 100 and preferably between 30 to 60
• Quality factor is high means the resistance is small and if
resistance is small, loss is also small
Sharpness of Tuning of Passive Filter

• Other type of passive shunt filters known as damped filter with a

high pass filter are tuned for a high frequencies and they have
low value of quality factor typically order of 0.5 to 5.
• These high pass passive filters offer low impedance over a
broad band of frequencies bandwidth.
• Cost of Passive Filter
• Cost of this passive filter is the reasonable and sometimes it
reaches 15% to 20% of the equipment for which it is used.
Therefore, the design of passive filter has to consider the cost in
account while designing the passive filter.
• Moreover, it has some power losses which also must be
considered in its design. The cost of passive filter may also be
partly supplemented to the reactive power supplied by it.
Shunt Active Power Filter

Current based power quality problems in distribution system

• Derating of the distribution system
• Distortion and voltage waveform at the point of common coupling
• Interference to communication system
• Disturbance to nearby consumers
Shunt Active Power Filter
The complete solution to above problems is the Shunt Active
Filter.

Shunt Active Power Filters are now matured technology

used for providing
✓harmonic current improvement
✓compensation of reactive power
✓load balancing
✓neutral current compensation
Shunt Active Power Filter
Shunt Active Power Filter (SAPF) are also used
✓ to regulate the terminal voltage
✓ to suppress the voltage flicker
✓ to improve the voltage balance in the 3-phase systems
Recent Developments
• varying configurations
• advanced control strategies
• utilize different solid-state devices
• Improved sensor technology like Hall Effect current sensor and
voltage sensor
• microelectronics revolution has reduced the cost of microcontrollers
and DSPs which have made it easier to implement the control for the
Shunt Active Filters
Shunt Active Power Filter

Industry made shunt active filter

Classification of Shunt Active Power Filter
CONVERTER TYPE
✓current source converter
✓voltage source converter
TOPOLOGY
✓half-bridge topology
✓full-bridge topology or H-bridge topology
NUMBER OF PHASES
✓two wire (single phase system)
✓three-wire (three-phase system)
✓four wire (three-phase system)
Converter Based Classification
• Current Fed Type Shunt APF

The drawback of this configurations is that the inductor is costly, bulky

and noisy. While, designing the DC inductor is not so easy.
Converter Based Classification
Voltage Fed Type Shunt APF

This is a shunt filter with a DC link capacitor. It is an electrolytic capacitor.

This makes the voltage fed type shunt active power filter cheap, less
losses, and small size.
 Topology Based Classification
• Half-bridge Topology of VSC based Single Phase Shunt Active
Power Filter.
• But the major
drawback of this is
that the current flow
through the capacitor
is large.
• Thus, size of the
capacitor or value of
capacitor is very high.
• It cannot be scaled for
large rating.
Topology Based Classification
• Full Bridge Topology of VSC Based Single-Phase Shunt Active
Filter
• It can be used at
higher rating.

• This bridge structure

have a many benefit
Supply System Based Classification
• Two Wire Shunt APF (CSI)
Supply System Based Classification

• Two Wire Shunt APF

Supply System Based Classification

• Three Wire Shunt APF With VSC

Supply System Based Classification
• Three-Phase Four-Wire Shunt Active Power Filter with
Capacitor Mid-Point Topology.
Supply System Based Classification

• Four-Pole, Four Wire Shunt APF

Supply System Based Classification

• Three H-Bridge, Four Wire Shunt APF

Operation of Shunt Active Filters
The main objective is mitigation of current based power quality
problems
✓Harmonics
✓Reactive power requirement
✓Unbalance current
✓Excessive neutral current.
KVA rating of the converter is undoubtedly increased to
accommodate for all the functionalities.
Requirements of Active Power Filter

✓A DC bus Capacitor
✓AC interacting inductors
✓Injection and Isolation Transformers
✓Small Passive Filters
Design of Three-Phase, Three-Leg VSC Based Active
Power Filter
 Design Parameters of SAPF
• Supply: A 3-pahse, 415 V, 50 Hz, with source resistance Rs=0.04 Ω,
source inductance Ls= 1mH, is considered.
• Load: A 3-phase, 3 wire rectifier is used as a non-linear load with
rectifier output current Id=224.17A.
• RMS value of rectifier input current= Irms=0.816× 224.17=182.92 A.
• Fundamental component value of rectifier input current (Il1 = 0.779 ×
224.17) = 174.78 A
2 2
• Harmonic Current= Ih = (𝐼𝑟𝑚𝑠 − 𝐼𝑙1 )= 55.86 A
Design Parameters of SAPF
• Rating of Shunt APF, 𝑃𝑓 = 3 × 𝑉𝑓 × 𝐼𝑓 = 3 × 239.6 × 55.86 =
40.152 kVA × 1.25 (25% Extra for dynamics) = 50.19 kVA.
• Hence, considering 50.19 kVA
• Shunt Active filter rating =50.19 kVA
• SAF voltage rating Vf= 415 V
• SAF current rating 𝐼𝑓 = 69.825 A
• Allowable Voltage Ripple at DC link Δ 𝑉𝑑𝑐 = 5% 𝑜𝑓 𝑉𝑑𝑐
• Allowable Ripple in SAF Current 𝐼𝑐𝑟(𝑝−𝑝) = 10% 𝑜𝑓 𝐼𝑓 = 6.983 𝐴
Design Parameters of SAPF
Selection of DC Bus Voltage
• 𝑉𝑑𝑐 = 2 2 𝑉𝐿𝐿 /( 3 𝑚)
• 𝑉𝑑𝑐 = 2 2 × 415/( 3 × 1)
• m is the modulation index and is assumed to be 1.
• Thus 𝑉𝑑𝑐 is obtained as 677.69 V for a 𝑉𝐿𝐿 of 415 V AC
distribution network.
• Hence, Let’s assume 𝑉𝑑𝑐 = 700 V
Design Parameters of SAPF

Selection of DC Capacitor
2 2
0.5× 𝐶𝑑𝑐 𝑉𝑑𝑐 − (𝑉𝑑𝑐1 ) =𝑘1 3𝑉𝑓 𝑎 𝐼𝑓 𝑡
• 𝑉𝑑𝑐1 = 1 − ∆𝑉𝑑𝑐 × 𝑉𝑑𝑐 = 1 − 0.05 × 700 = 665 𝑉
• 0.5 × 𝐶𝑑𝑐 7002 − 6652 = 0.1 × 3 × 239.6 × 1.2 × 69.125 × 0.03
• (𝑘1 = 0.1, 𝑎 = 1.2, 𝑡 = 30𝑚𝑠)
• 𝐶𝑑𝑐 = 7564 𝜇𝐹
• Hence, 𝑉𝑑𝑐 is the nominal DC voltage, 𝑉𝑑𝑐1 is the minimum voltage
level of the DC bus, ‘a’ is the overloading factor, and t is the time by
which the DC bus voltage is to be recovered
• The calculated value of 𝐶𝑑𝑐 = 7564 𝜇𝐹 and it is selected as 7600 𝜇𝐹
 Design Parameters of SAPF
Selection of AC Inductor:

• 𝐿𝑟 = (√3 m 𝑉𝑑𝑐 )/(12𝑎𝑓𝑠 𝐼𝑐𝑟(𝑝−𝑝) )

3×1×700
• 𝐿𝑟 = (a=1.2, 𝑓𝑠 =10kHz, 𝐼𝑐𝑟(𝑝−𝑝) =6.983A)
12×1.2×10000×6.983

• 𝐿𝑟 =1.206 mH

• m is the modulation index and a is the overloading factor

• The 𝐿𝑟 may be selected as 1.5 mH

Design Parameters of SAPF

• Selection of a Ripple Filter

• To filter out the noise from the voltage at PCC
 Design Parameters of SAPF
• Selection of Ripple Filter
Considering 𝑅𝑓 = 10𝛺 𝑎𝑛𝑑 𝐶𝑓 = 5.5𝜇𝐹
➢For the Fundamental frequency (𝑓 = 50𝐻𝑧)
➢𝑍𝑓 = (10)2 + 1/(2𝜋 × 50 × 5.5 × 10−6 ) 2 =578.832 Ω
➢For switching frequency (𝑓𝑠 = 10 𝑘𝐻𝑧)
➢𝑍𝑓 = (10)2 + 1/(2𝜋 × 10000 × 5.5 × 10−6 ) 2 =10.410 Ω
➢From the above it can be seen that the impedance offered for
switching frequency 10kHz is 10.410 Ω and for fundamental
frequency is 578.832 Ω, which is sufficiently large and hence the
ripple filter draws negligible fundamental frequency current
Design Parameters of SAPF

• Selection of Voltage Rating of the Solid State Switches

• 𝑽𝒔𝒘 = 𝑽𝒅𝒄 + 𝑽𝒅
• Where 𝑉𝑑 is the 10% overshoot in the DC link voltage under
dynamic condition
• 𝑽𝒔𝒘 = 𝑽𝒅𝒄 + 𝑽𝒅 = 𝑽𝒅𝒄 +0.1% of 𝑽𝒅𝒄
• 𝑽𝒔𝒘 =700+70=770 V
• With an appropriate safety factor the voltage rating may be
selected as 1200 V, IGBTs for the VSC used in the SAPF
Design Parameters of SAPF
• Selection of Current Rating of the Solid State Switches
• 𝐼𝑆𝑊 = 1.25(𝐼𝑐𝑟(𝑝−𝑝) + 𝐼𝑓(𝑝−𝑝) )
• Where 𝐼𝑐𝑟(𝑝−𝑝) =0.1 × 𝐼𝑓 =0.1 ×69.83=6.983 A
• 𝐼𝑓(𝑝−𝑝) =√2 × 𝐼𝑓 =√2 × 69.83=98.755 A
• 𝐼𝑆𝑊 =1.25(6.983+98.755) = 132.172 A
• Thus the solid state switch (IGBT) for the VSC will be seleted with
next higher available rating of 1200 V and 300 A.