Unit 6 power quality

UNIT-6
POWER QUALITY MONITORING
Department of Electrical & Electronics Engineering
Electrical Power Quality
By
HARI MADHAVA REDDY .Y
Assistant Professor

6. POWER QUALITY MONITORING
6.1. Monitoring Considerations
6.2. Historical Perspective of Power Quality Measuring
Instruments
6.3. Power Quality Measurement Equipment
6.4. Assessment of Power Quality Measurement Data
6.5. Application of Intelligent Systems
6.6. Power Quality Monitoring Standards
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6.1.1. Monitoring as part of a facility site survey
6.1.2. Determining what to monitor
6.1.3. Choosing monitoring locations
6.1.4. Options for permanent power quality monitoring
equipment
6.1.5. Disturbance monitor connections
6.1.6. Setting monitor thresholds
6.1.7. Quantities and duration to measure
6.1.8. Finding the source of a disturbance

Power quality monitoring is the process of gathering, analyzing,
and interpreting raw measurement data into useful information.
The process of gathering data is usually carried out by
continuous measurement of voltage and current over an extended
period. The process of analysis and interpretation has been
traditionally performed manually, but recent advances in signal
processing and artificial intelligence fields have made it possible to
design and implement intelligent systems to automatically analyze and
interpret raw data into useful information with minimum human
intervention.
The monitoring objectives often determine the choice of monitoring
equipment, triggering thresholds, methods for data acquisition and
storage, and analysis and interpretation requirements.
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Need for power quality monitoring:
☺Power quality monitoring programs improving the system wide
power quality performance. Many industrial and commercial
customers have equipment that is sensitive to power disturbances,
and, therefore, it is more important to understand the quality of
power being provided.
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Sensitive to PQ disturbance
 Computer networking.
 Telecommunication facilities.
 Semi- conductor and electronics manufacturing facilities.
 Biotechnology and pharmaceutical laboratories.
 Financial data-processing centres.

6.1.1.Common objectives of power quality monitoring
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 Monitoring to characterize system performance.
Need to understand its system performance and then match that system
performance with the needs of customers
 Monitoring to characterize specific problems.
Performing short-term monitoring at specific customer sites or at difficult
loads.
 Monitoring as part of an enhanced power quality service.
A provider and customer can together achieve this goal by modifying the power
system or by installing equipment within the customer’s premises.
 Monitoring as part of predictive or just-in-time maintenance
Equipment maintenance can be quickly ordered to avoid catastrophic failure.

Monitoring as part of a facility site survey
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1. Nature of the problems (data loss, nuisance trips, component fail- ures,
control system malfunctions, etc.)
2. Characteristics of the sensitive equipment experiencing problems
(equipment design information or at least application guide information)
3. The times at which problems occur
4. Coincident problems or known operations (e.g., capacitor switching)that
occur at the same time
5. Possible sources of power quality variations within the facility (motor
starting, capacitor switching, power electronic equipment operation,
arcing equipment, etc.)
6. Existing power conditioning equipment being used.
7. Electrical system data (one-line diagrams, transformer sizes and
impedances, load information, capacitor information, cable data, etc.)

6.1.2. Determining what to monitor
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 Power quality encompasses a wide variety of conditions on the
power system. Important disturbances can range from very
high frequency impulses caused by lightning strokes or current
chopping during circuit interruptions to long-term over voltages
caused by a regulator tap switching problem.
 The range of conditions that must be characterized creates
challenges both in terms of the monitoring equipment performance
specifications and in the data-collection requirements.

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The methods for characterizing the quality of ac power are
important for the monitoring requirements. For instance, characterizing
most transients requires high-frequency sampling of the actual
waveform. Voltage sags can be characterized with a plot of the rms
voltage versus time.
Outages can be defined simply by a time duration. Monitoring
to characterize harmonic distortion levels and normal voltage
variations requires steady-state sampling with results analysis of trends
over time.
Extensive monitoring of all the different types of power quality
variations at many locations may be rather costly in terms of
hardware, communications charges, data management, and report
preparation. Hence, the priorities for monitoring should be determined
based on the objectives of the effort.

6.1.3. Choosing monitoring locations
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 The primary objective was to characterize power quality on
primary distribution feeders, monitoring was done on the actual
feeder circuits. One monitor was located near the substation, and
two additional sites were selected randomly.
 Monitoring locations should be at actual customer service entrance
locations because it includes the effect of step-down transformers
supplying the customer.
 Locate the monitors as close as possible to the equipment affected
by power quality variations.
 Monitor at the substation and at selected customer service
entrance locations. The substation is important because it is the
PCC for most rms voltage variations.

11 Figure: Typical distribution feeder monitoring scheme.

6.1.4. Options for permanent power
quality monitoring equipment
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Permanent power quality monitoring systems should take
advantage of the wide variety of equipment that may have the
capability to record power quality information.
Some of the categories of equipment that can be incorporated
into an overall monitoring system include the following:
1. Digital fault recorders (DFRs).
2. Smart relays and other IEDs.
3. Voltage recorders.
4. In-plant power monitors.
5. Special-purpose power quality monitors
6. Revenue meters

13 Figure: Illustration of system power quality monitoring concept with monitoring at the
substation and selected customer locations.

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1. Digital fault recorders (DFRs):
DFR will typically trigger on fault events and record the
voltage and current waveforms that characterize the event. This makes
them valuable for characterizing rms disturbances, such as voltage
sags, during power system faults. DFRs also offer periodic waveform
capture for calculating harmonic distortion levels.
2. Smart relays and other IEDs.
Many types of substation equipment may have the capability to
be an intelligent electronic device (IED) with monitoring capability.
Manufacturers of devices like relays and reclosers that monitor the
current anyway are adding on the capability to record disturbances and
make the information available to an overall monitoring system
controller. These devices can be located on the feeder circuits as well
as at the substation.

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3. Voltage recorders
The voltage recorder provides a trend that gives the maximum,
minimum, and average voltage within a specified sampling window
(for example, 2 s). With this type of sampling, the recorder can
characterize a voltage sag magnitude adequately. However, it will not
provide the duration with a resolution less than 2 s.
4. In-plant power monitors
Particularly those located at the service entrance, can be used as
part of a utility monitoring program. Capabilities usually include wave
shape capture for evaluation of harmonic distortion levels, voltage
profiles for steady-state rms variations, and triggered wave shape
captures for voltage sag conditions.
6. Revenue meters.
Revenue meters monitor the voltage and current anyway, so it
seems logical to offer alternatives for more advanced monitoring that
could include recording of power quality information.

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5. Special-purpose power quality monitors.
The monitoring instrument developed for the EPRI DPQ project
was specifically designed to measure the full range of power quality
variations.
This instrument features monitoring of voltage and current on
all three phases plus the neutral. A 14-bit analog-to-digital (A/D) board
provides a sampling rate of 256 points per cycle for voltage and 128
points per cycle for current. This high sampling rate allowed detection
of voltage harmonics as high as the 100th and current harmonics as
high as the 50th.
Most power quality instruments can record both triggered
and sampled data. Triggering should be based upon rms thresholds for
rms variations and on wave shape for transient variations. Simultaneous
voltage and current monitoring with triggering of all channels during a
disturbance is an important capability for these instruments. Power
quality monitors have proven suitable for substations, feeder locations,
and customer service entrance locations.

6.1.5. Disturbance monitor connections
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Some manufacturers include input filters and/or surge
suppressors on their power supplies that can alter disturbance data if
the monitor is powered from the same circuit that is being monitored.
The disturbance monitor will have a ground connection for the
signal to be monitored and a ground connection for the power sup-
ply of the instrument. Both of these grounds will be connected to the
instrument chassis. For safety reasons, both of these ground terminals
should be connected to earth ground.

6.1.6 Setting monitor thresholds
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Disturbance monitors are designed to detect conditions that are
abnormal. Therefore, it is necessary to define the range of conditions
that can be considered normal. Some disturbance monitors have
preselected (default) thresholds that can be used as a starting point. The
best approach for selecting thresholds is to match them with the
specifications of the equipment that is affected.
The thresholds are essentially fixed in the instruments, and
algorithms may be adjusted internally based on the disturbances
being recorded. This type of system is convenient for the user since
the setup is simple, but it is still a compromise since you lose the
capability to change the thresholds based on the specific
circumstances at a particular location.

6.1.7 Quantities and duration to measure
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When characterizing system disturbances, it is sufficient to
monitor only the voltage signals. Voltages provide information about
the quality of power being delivered to a facility and characterize the
transients and voltage sags that may affect customer equipment.
A tremendous amount of information in the currents associated
with these disturbances that can help determine the cause and
whether or not equipment was impacted. Also, current measurements
are required if harmonics are a concern since the currents characterize
the harmonic injection from the customer onto the power system.

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Current measurements are used:
 To characterize the generation of harmonics by nonlinear
loads.
 At individual loads are valuable for determining these
harmonic generation characteristics.
 On feeder circuits or at the service entrance characterize a
group of loads or the entire facility as a source of harmonics.
 On the distribution system can be used to characterize groups
of customers or an entire feeder
Voltage measurements are used:
 System response to the generated harmonic currents.
 Resonance conditions will be indicated by high harmonic
voltage distortion at specific frequencies
 Frequency response characteristics from measurements
The duration of monitoring depends on the monitoring objectives.

6.1.8 Finding the source of a disturbance
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The first step in identifying the source of a disturbance is to
correlate the disturbance waveform with possible causes. The
following general guidelines can help:
■ High-frequency voltage variations will be limited to locations
close to the source of the disturbance. Low-voltage (600 V and
below) wiring often damps out high-frequency components very
quickly due to circuit resistance, so these frequency components
will only appear when the monitor is located close to the source of
the disturbance.

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■ Power interruptions remote from the monitoring location will
result in a decaying voltage due to stored energy in rotating
equipment and capacitors.
■ The highest harmonic voltage distortion levels will occur close
to capacitors that are causing resonance problems . In these cases, a
single frequency will usually dominate the voltage harmonic
spectrum.

6.2. Historical Perspective of Power
Quality Measuring Instruments
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Early monitoring devices were bulky, heavy boxes that
required a screw- driver to make selections. One of the earliest power
quality monitoring instruments is a lightning strike recorder
developed by General Electric in the 1920s
Figure Early power quality
instrument developed by GE in
the 1920s to record lightning
strikes.
Significant development on power
quality devices was not made until the
1960s when Martzloff developed a surge
counter that could capture a voltage
waveform of lightning strikes.

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Figure : Martzloff ’s 1967
surge counter. Recorded data
are quantitative
The mid-1970s when Dranetz Engineering
Laboratories (now Dranetz-BMI)
introduced the Series 606 power line
disturbance analyzer . This was a
microprocessor- based monitor- analyzer
first manufactured in 1975. The output of
these monitors was text-based, printed on
a paper tape.
Second-generation power quality
instruments debuted in the mid-1980s.
This generation of power quality monitors
generally featured full graphic display and
digital memory to view and store
captured power quality events, including
both transients and steady-state events.

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By the mid-1990s, the third-generation power quality instruments
emerged. The development of the third-generation power monitors
was inspired in part by the EPRI DPQ (Electric Power Research
Institute Distribution System Power Quality Monitoring ) project.
1. Managing the large volume of raw measurement data that must
be collected, analyzed, and archived becomes a serious
challenge as the number of monitoring points grows.
2. The data volume collected at each monitoring point can strain
communication mechanisms employed to move that data from
monitor to analysis point.
3. As understanding of system performance grows through the
feed- back provided by the monitoring data, detailed views
of certain events, such as normal capacitor switching, become
less valuable and would be of more use in a summary or
condensed form.
4. The real value of any monitoring system lies in its ability to
generate information rather than in collecting and storing
volumes of detailed raw data.

6. 3 Power Quality Measurement
Equipment
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Basic categories of instruments that may be applicable include
 Wiring and grounding test devices
 Multi meters
 Oscilloscopes
 Disturbance analysers
 Harmonic analysers and spectrum analysers
 Combination disturbance and harmonic analysers
 Flicker meters
 Energy monitors

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Other instruments that can be used to help solve power quality
problems by measuring ambient conditions:
 Infrared meters can be very valuable in detecting loose
connections and overheating conductors.
 Noise problems related to electromagnetic radiation may
require measurement of field strengths in the vicinity of affected
equipment. Magnetic gauss meters are used to measure magnetic
field strengths for inductive coupling concerns. Electric field
meters can measure the strength of electric fields for
electrostatic coupling concerns.
 Static electricity meters are special-purpose devices used to
measure static electricity in the vicinity of sensitive equipment.

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Regardless of the type of instrumentation needed for a particular test,
there are a number of important factors that should be considered
when selecting the instrument. Some of the more important factors
include
Ø 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

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Wiring and grounding test devices
Many power quality problems reported by end users are caused by
problems with wiring and/or grounding within the facility. These
problems can be identified by visual inspection of wiring,
connections, and panel boxes and also with special test devices for
detecting wiring and grounding problems.
Important capabilities for a wiring and grounding test device include
Ø Detection of isolated ground shorts and neutral-ground bonds
Ø Ground impedance and neutral impedance measurement or indication
Ø Detection of open grounds, open neutrals, or open hot wires
Ø Detection of hot/neutral reversals or neutral/ground reversals

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Three-phase wiring testers should also test for phase rotation and phase-
to-phase voltages. These test devices can be quite simple and provide an
excellent initial test for circuit integrity. Many problems can be detected
without the requirement for detailed monitoring using expensive
instrumentation.
Multimeters:
After initial tests of wiring integrity, it may also be necessary
to make quick checks of the voltage and/or current levels within a
facility. Overloading of circuits, under voltage and overvoltage
problems, and unbalances between circuits can be detected in this
manner. These measurements just require a simple multi meter.
Signals used to check for these include

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Ø Phase-to-ground voltages
Ø Phase-to-neutral voltages
Ø Neutral-to-ground voltages
Ø Phase-to-phase voltages (three-phase system)
Ø Phase currents
Ø Neutral currents
The most important factor to consider when selecting and using
a multimeter is the method of calculation used in the meter. All the
commonly used meters are calibrated to give an rms indication for the
measured signal. However, a number of different methods are used to
calculate the rms value. The three most common methods are

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1. Peak method: Assuming the signal to be a sinusoid, the meter reads
the peak of the signal and divides the result by 1.414 (square root of 2) to
obtain the rms.
2. Averaging method: The meter determines the average value of a
rectified signal. For a clean sinusoidal signal (signal containing only one
frequency), this average value is related to the rms value by a constant.
3. True rms: The rms value of a signal is a measure of the heating that
will result if the voltage is impressed across a resistive load. One method
of detecting the true rms value is to actually use a thermal detector to
measure a heating value. More modern digital meters use a digital
calculation of the rms value by squaring the signal on a sample by-
sample basis, averaging over the period, and then taking the square root
of the result. These different methods all give the same result for a clean,
sinusoidal signal but can give significantly different answers for distorted
signals.

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Oscilloscopes
An oscilloscope is valuable when performing real-time tests.
Looking at the voltage and current waveforms can provide
much information about what is happening, even without
performing detailed harmonic analysis on the waveforms. One can
get the magnitudes of the voltages and currents, look for obvious
distortion, and detect any major variations in the signals.

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Disturbance analyzers
Disturbance analyzers and disturbance monitors form a
category of instruments that have been developed specifically for
power quality measurements. They typically can measure a wide
variety of system disturbances from very short duration transient
voltages to long-duration outages or under voltages. Thresholds can be
set and the instruments left unattended to record disturbances over a
period of time. The information is most commonly recorded on a
paper tape, but many devices have attachments so that it can be
recorded on disk as well. There are basically two categories of these
devices:
1. Conventional analyzers that summarize events with specific
information such as overvoltage and undervoltage magnitudes, sags
and surge magnitude and duration, transient magnitude and duration,
etc.
2. Graphics-based analyzers that save and print the actual waveform
along with the descriptive information which would be generated by
one of the conventional analyzers

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It is often difficult to determine the characteristics of a disturbance or a transient
from the summary information available from conventional disturbance analyzers.
For instance, an oscillatory transient cannot be effectively described by a peak and a
duration. Therefore, it is almost imperative to have the waveform capture capability
of a graphics-based disturbance analyzer for detailed analysis of a power quality
problem (Fig. 5.2). However, a simple conventional disturbance monitor can be
valuable for initial checks at a problem location.

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Spectrum analyzers and harmonic analyzers
Harmonic analyzers have several capabilities. They capture harmonic
waveforms and display them on a screen. They calculate the K factor
to de rate transformers and the total harmonic distortion (THD) in
percent of the fundamental. They also measure the
corresponding frequency spectrum, i.e., the harmonic frequency
associated with the current and voltage up to the fiftieth harmonic.
They display the harmonic frequency on a bar graph or as the signal’s
numerical values.
Some measure single-phase current and voltage while others measure
three-phase current and voltage. All of them measure the power factor
(PF). The power factor provides a measurement of how much of the
power is being used efficiently for useful work. Some can store data
for a week or more for later transfer to a PC for analysis.

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This makes them powerful tools in the analysis of harmonic power quality
problems. Some of the more powerful analyzers have add-on modules that can be
used for computing fast Fourier transform (FFT) calculations to determine the lower-
order harmonics. However, any significant harmonic measurement requirements will
demand an instrument that is designed for spectral analysis or harmonic analysis.
Important capabilities for useful harmonic measurements include Capability to
measure both voltage and current simultaneously so that harmonic power flow
information can be obtained.
■ Capability to measure both magnitude and phase angle of individual harmonic
components (also needed for power flow calculations).
■ Synchronization and a sampling rate fast enough to obtain accurate measurement
of harmonic components up to at least the 37th harmonic (this requirement is a
combination of a high sampling rate and a sampling interval based on the 60-Hz
fundamental).
■ Capability to characterize the statistical nature of harmonic distortion levels
(harmonics levels change with changing load conditions and changing system
conditions).

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1. Simple meters. It may sometimes be necessary to make a quick check of
harmonic levels at a problem location. A simple, portable meter for this purpose is
ideal. There are now several hand-held instruments of this type on the market. Each
instrument has advantages and disadvantages in its operation and design. These
devices generally use microprocessor-based circuitry to perform the necessary
calculations to determine individual harmonics up to the 50th harmonic, as well as the
rms, the THD, and the telephone influence factor (TIF). Some of these devices can
calculate harmonic powers (magnitudes and angles) and can upload stored waveforms
and calculated data to a personal computer.
2. General-purpose spectrum analyzers. Instruments in this category are designed
to perform spectrum analysis on waveforms for a wide variety of applications. They
are general signal analysis instruments. The advantage of these instruments is that they
have very powerful capabilities for a reasonable price since they are designed for a
broader market than just power system applications. The disadvantage is that they are
not designed specifically for sampling power frequency waveforms and, therefore,
must be used carefully to assure accurate harmonic analysis. There are a wide variety
of instruments in this category.
There are basically three categories of instruments to consider for
harmonic analysis:

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3. Special-purpose power system harmonic analyzers. Besides the general-
purpose spectrum analyzers just described, there are also a number of instruments
and devices that have been designed specifically for power system harmonic
analysis. These are based on the FFT with sampling rates specifically designed for
determining harmonic components in power signals. They can generally be left in
the field and include communications capability for remote monitoring.
Flicker meters
RMS strip charts. Historically, flicker has been measured using rms meters, load
duty cycle, and a flicker curve. If sudden rms voltage deviations occurred with
specified frequencies exceeding values found in flicker curves, such as one shown in
Fig. 11.16, the system was said to have experienced flicker. A sample graph of rms
voltage variations is shown in Fig. 11.17 where large voltage deviations up to 9.0 V
rms ( V/V± 8.0 percent on a 120-V base) are found.

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Fast Fourier transforms. Another method that has been used to measure flicker is to
take raw samples of the actual voltage waveforms and implement a fast Fourier
transform on the demodulated signal (flicker signal only) to extract the various
frequencies and magnitudes found in the data. These data would then be compared to
a flicker curve. Although similar to using the rms strip charts, this method more
accurately quantifies the data measured due to the magnitude and frequency of the
flicker being known.

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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.
Generally, 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.

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The most established method for doing this is described in IEC
Standard 61000-4-15. The IEC flicker meter consists of five
blocks, which are shown in above Fig.
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 behaviour of the
incandescent lamp.
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.
The basic transfer function for the weighting filter is

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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.
Flicker level evaluation can be divided into two categories, short-
term and long-term. Short-term evaluation of flicker severity PST is
based upon an observation period of 10 min. This period is based upon
assessing disturbances with a short duty cycle or those that produce
continuous fluctuations. PST can be found using the equation

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where the percentages P0.1, P1s, P3s, P10s, and P50s are the flicker
levels that are exceeded 0.1, 1.0, 3.0, 10.0, and 50.0 percent of the
time, respectively. These values are taken from the cumulative
distribution curve discussed previously. A PST of 1.0 on the output of
block 5 represents the objectionable (or irritable) limit of flicker.
The long-term flicker severity PLT is derived from PST using the
equation
where N is the number of PST readings and is determined by the duty
cycle of the flicker-producing load.

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Smart power quality monitors
All power quality measurement instruments previously described are
designed to collect power quality data. Some instruments can send the
data over a telecommunication line to a central processing location for
analysis and interpretation. However, one common feature among
these instruments is that they do not possess the capability to locally
analyze, interpret, and determine what is happening in the power sys-
tem. They simply record and transmit data for post processing.
Since the conclusion of the EPRI DPQ project in Fall 1995, it was
realized that these monitors, along with the monitoring practice
previously described, were inadequate. An emerging trend in power
quality monitoring practice is to collect the data, turn them into
useful information, and disseminate it to users. All these processes
take place within the instrument itself. Thus, a new breed of power
quality monitor was developed with integrated intelligent systems to
meet this new challenge.

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Transducer requirements
Monitoring of power quality on power systems often requires
transducers to obtain acceptable voltage and current signal levels.
Voltage monitoring on secondary systems can usually be performed
with direct connections, but even these locations require current
transformers (CTs) for the current signal.
Two important concerns must be addressed in selecting transducers:
1. Signal levels: Signal levels should use the full scale of the
instrument without distorting or clipping the desired signal.
2. Frequency response: This is particularly important for transient
and harmonic distortion monitoring, where high-frequency signals are
particularly important.

6.4. Assessment of Power Quality Measurement Data
Standards
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There are two streams of power quality data analysis, i.e.,
off-line and on-line analyses.
The off-line power quality data analysis is performed off-
line at the central processing locations.
The on-line data analysis is performed within the instrument
itself for immediate information dissemination.
Off-line power quality data assessment:
Off-line power quality data assessment is carried out separately from
the monitoring instruments. Dedicated computer software is used for
this purpose.

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Large-scale monitoring projects with large volumes of data to
analyze often present a challenging set of requirements for software
designers and application engineers.
The new standard format for interchanging power quality data the
Power Quality Data Interchange Format (PQDIF) makes sharing of
data between different types of monitoring systems much more
feasible. This means that applications for data management and
data analysis can be written by third parties and measurement data
from a wide variety of monitoring systems can be accessible to these
systems. PQ View (www.pqview.com) is an example of this type of
third-party application.

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The off-line power quality data assessment software usually performs
the following functions:
■ Viewing of individual disturbance events.
■ RMS variation analysis which includes tabulations of voltage sags
and swells, magnitude-duration scatter plots based on CBEMA, ITI, or
user-specified magnitude-duration curves, and computations of a wide
range of rms indices such as SARFI, SIARFI, and CAIDI.
■ Steady-state analysis which includes trends of rms voltages, rms cur-
rents, and negative- and zero-sequence unbalances. In addition, many
software systems provide statistical analysis of various minimum,
average, maximum, standard deviation, count, and cumulative
probability levels. Statistics can be temporally aggregated and
dynamically filtered. Figures 11.26 and 11.27 show the time trend of
phase A rms voltage along with its histogram representation.
■ Harmonic analysis where users can perform voltage and current
harmonic spectra, statistical analysis of various harmonic indices, and
trending overtime.

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Transient analysis which includes statistical analysis of maximum
voltage, transient durations, and transient frequency Standardized
power quality reports (e.g. daily reports, monthly reports, statistical
performance reports, executive summaries, customer power quality
summaries).
■ Analysis of protective device operation (identify problems).
■ Analysis of energy use.
■ Correlation of power quality levels or energy use with
important parameters (e.g., voltage sag performance versus
lightning flash density).
■ Equipment performance as a function of power quality levels
(equipment sensitivity reports).

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On-line power quality data assessment
On-line power quality data assessment analyses data as they are
captured. The analysis results are available immediately for rapid
dissemination. Complexity in the software design requirement for
on-line assessment is usually higher than that of off-line.
One of the primary advantages of on-line data analysis is that it
can provide instant message delivery to notify users of specific
events of interest. Users can then take immediate actions upon
receiving the notifications.

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Figure 11.28 illustrates a simple message delivered to a user reporting that a
capacitor bank located upstream from a data acquisition node called “Data Node
H09_5530” was energized at
05-15-2002 at 04:56:11 A.M. The message also details the transient
characteristics such as the magnitude, frequency, and duration along with the
relative location of the capacitor bank from the data acquisition node.

6.5. Application of Intelligent Systems
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 Many advanced power quality monitoring systems are equipped with either off-
line or on-line intelligent systems to evaluate disturbances and system
conditions so as to make conclusions about the cause of the problem or even
predict problems before they occur.
 These helps engineers determine the system condition rapidly.
 It significantly increase the value of a monitoring application since it can
generate information rather than just collect data.
 The intelligent systems are packaged as individual autonomous expert system
modules, where each module performs specific functions. Examples include an
expert system module that analyzes capacitor- switching transients and
determines the relative location of the capacitor bank, and an expert system
module to determine the relative location of the fault causing a voltage sag.

6.6. Power Quality Monitoring Standards
60
IEEE 1159: Guide for power quality monitoring
It developed to provide general guidelines for power quality measurements and to
provide standard definitions for the different categories of power quality problems.
The IEEE 1159.1 Working Group is developing guidelines for instrumentation
requirements associated with different types of power quality phenomena. These
requirements address issues like sampling rate requirements, synchronization, A/D
sampling accuracy, and number of cycles to sample.
The IEEE 1159.2 Working Group is developing guidelines for characterizing
different power quality phenomena. This includes definition of important
characteristics that may relate to the impacts of the power quality variations (such
as minimum magnitude, duration, phase shift, and number of phases for voltage
sags).

61
The IEEE 1159.3 Working Group is defining an interchange format that can be
used to exchange power quality monitoring information between different
applications. IEEE developed the COMTRADE format for exchanging waveform
data between fault recorders and other applications, such as relay testing
equipment.
IEC 61000-4-30: Testing and Measurement Techniques—Power Quality
Measurement Methods
IEC 61000-4-7 provides the specifications for monitoring harmonic distortion
levels. IEC
61000-4-15 provides the specifications for monitoring flicker, as previously
described.

Questions/Discussions
 Explain importance of PQ Monitoring and
considerations.
 Explain the various power quality measuring
equipment. Briefly explain modern power quality
analyzer?
 Explain the application of expert system for power
quality monitoring.
 Explain about historical perspective of PQ
measuring instruments
62

Unit 6 power quality

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Unit 6 power quality