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
• 𝐼𝐼𝐻𝐻 = √(𝐼𝐼22 + 𝐼𝐼32 + 𝐼𝐼42 + 𝐼𝐼52 + ⋯
• 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 increases 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 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 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
Numerical Example
• In a square wave of current I of 100A, calculate (a) the crest factor CF
(b) Distortion factor (DF) and (c) total harmonic distortion % (THD)
Solution: For given I=100A
The rms of the fundamental component is I1= (2√2/π) × I= 0.9×I
The rms value of the square wave Irms= I
CF= Peak Value/RMS value of the square wave= I/I=1
DF= Rms value of fundamental component of square wave/ RMS
value of square wave.
DF=I1/I= 0.9×I/I=0.9
THD of square wave=√(I²rms-I1²)/I1= √{I²-(0.9×I²)}/0.9×I=48.43%
• For a quasi-square wave (120° pulse width) of current with an
amplitude I of 100A (shown in Figure), calculate (a) crest factor (CF),
(b) distortion factor (DF), and (c) total harmonic distortion (THD)
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 (PST),
as well as the long-term flicker severity (PLT).
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
RMS Voltage variation
Voltage Flicker waveform
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
