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Power Quality Problems,
Power System Protection and Solution:
Theory and Case Studies
1. PQ Disturbances and Analysis
6-8 Sept 2022 @ The Sukosol Hotel
Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
Power System Research Lab
Dept. Electrical Engineering, Chulalongkorn University
Email: Thavatchai.t@chula.ac.th
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1: PQ Disturbances and Analysis
Section Overview
1.1 Power Quality Definitions
1.2 Classification of PQ Disturbances
1.3 PQ Disturbances Analysis: How to analyze PQ
disturbances and analysis tools
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1: PQ Disturbances and Analysis
Overview
1.1 Power Quality Definitions
• IEC 61000-4-30 (2015) [1] => Characteristics of the
electricity at a given point on an electrical system,
evaluated against a set of reference technical
parameters
• IEEE 1159 (2019) [2] => A wide variety of
electromagnetic phenomena that characterize the
voltage and current at a given time and at a given
location on the power system
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
• IEC 61000-2-5 (2011) [3]
Three categories of electromagnetic environment phenomena
1. Electrostatic discharge phenomena (conducted and radiated)
2. Low-frequency phenomena (conducted and radiated)
3. High-frequency phenomena (conducted and radiated)
Radiated disturbances occur in the medium surrounding the
equipment, while conducted disturbances occur in various
metallic media.
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
Low-frequency conducted phenomena
- Harmonics ✓
- Voltage variations ✓
- Voltage dips ✓
- Voltage unbalance ✓
- Voltage interruptions
- Voltage frequency variations
- DC in AC networks
High-frequency conducted phenomena
- Unidirectional and oscillatory transients
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1: PQ Disturbances and Analysis
IEC 61000-2-5
1. Low Frequency
Phenomena (f < 9 kHz)
1.1 Conducted
2. High Frequency
Phenomena (f > 9 kHz)
1.2 Radiated
2.1 Conducted
1) Harmonics and
interharmonics
1) Magnetic fields
2) Signalling voltages
2) Electric fields
3) Voltage fluctuations
1) Induced
continuous wave
voltages and
currents
2) Unidirectional
transients
4) Voltage dips and interruptions
3. Electrostatic
discharge
(ESD)
2.2 Radiated
1) Magnetic fields
2) Electric fields
3) Electromagnetic fields:
Continuous wave and
Transient
3) Oscillatory transients
5) Voltage unbalance
6) Power frequency variations
7) Induced low frequency voltages
8) DC in AC networks
Electromagnetic disturbances
classification based on IEC 61000-2-5
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
• IEEE 1159 (2019) [2]: Seven categories of electromagnetic
phenomena
1. Transients: Impulsive and Oscillatory transients
2. Short-duration rms variations: Interruption, Sag ✓ and Swell
3. Long-duration rms variations: Interruption, Overvoltage ✓,
Undervoltage and Current Overload
4. Imbalance ✓
5. Waveform distortion: DC offset, Harmonics ✓,
Interharmonics, Notching and Noise
6. Voltage fluctuation ✓
7. Power frequency variations
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1.2 Classification of PQ Disturbances
Typical Waveforms of PQ Disturbances [2]
Oscillatory transient from cap-bank energization
Momentary interruption due to fault
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
Typical Waveforms of PQ Disturbances [2]
Voltage sag caused by a single line-to-ground fault
Voltage sag caused by motor starting
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
Typical Waveforms of PQ Disturbances [2]
Voltage swell caused by a single line-to-ground fault
Voltage imbalance for residential feeder
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
Typical Waveforms of PQ Disturbances [2]
Current waveform and harmonic spectrum for
adjustable speed drive (ASD) input current
Voltage fluctuation caused by arc
furnace operation
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1: PQ Disturbances and Analysis
Supraharmonics
Supraharmonic emissions are a new phenomenon in the
electrical grid integrated into renewable energy sources and
can be characterized as the harmonics distortion with a
frequency range from 2 kHz to 150 kHz.
High-frequency switching equipment is the source of
numerous high-frequency disturbances. Examples are
• Chargers for electric cars
• Inverters of PV systems
• Frequency drives
• Switching power supplies (e.g. Street LED lamps)
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
Source: Ammar Ahmed Alkahtani, et.al Power Quality in Microgrids Including Supraharmonics: Issues,
Standards, and Mitigations, IEEE Access, Vol. 8, 2020
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1: PQ Disturbances and Analysis
1.2 Classification of PQ Disturbances
Observations
1. Different PQ disturbances have different waveforms.
This will help identifying PQ disturbances from captured
waveforms from monitoring.
2. Characteristics of PQ disturbances are not the same. As
a result, different analysis methods should be applied
and different mitigations are required.
3. Waveforms alone might not help getting the whole
picture about PQ disturbances. RMS plots are also
needed. However, more analyses can be further carried
out using waveforms of PQ disturbances.
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1.3 PQ Disturbances Analysis
1. Transients:
1.1) Impulsive: Waveform analysis with rise/decay time and
spectral content, e.g. 1.2/50 s
1.2) Oscillatory: Waveform analysis with magnitude,
duration and spectral content (primary frequency component)
High-freq. oscillatory transient: primary freq. > 500 kHz
Medium-freq. oscillatory transient: 5 kHz < primary freq. < 500 kHz
Low-freq. oscillatory transient: primary freq. < 5 kHz
*** Transient current is often a better measure of the severity of
a.c. system transients than the transient voltage [1].
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1.3 PQ Disturbances Analysis
2. Voltage Sag, Swell and Interruption: Waveforms and RMS
plots with magnitude and duration (One-cycle RMS values
updated each half-cycle [1].)
3. Overvoltage and Undervoltage: Waveforms and RMS plots
with magnitude and duration (longer than 1 min. [2])
4. Harmonics and Interharmonics: Waveforms using Fourier
analysis (1-cycle and multiple-cycle windows) with important
Harm/Interharm magnitudes, phase angle and other indices
such as Individual Harmonic and Total Harmonic Distortion (THD)
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1.3 PQ Disturbances Analysis
5. Voltage Imbalance or Unbalance: Waveforms and RMS
plots with voltage imbalance index (Many definitions are
available, but the most reliable one is the negative-to-positive
sequence ratio [2].)
6. Voltage fluctuation: Waveforms and RMS plots with
normalized voltage change index (V/V), modulation
frequency and flicker severity indices such as Pst (short-term
in 10-min period) and Plt (Long-term in 2-hrs period).
*** Flicker is derived from the impact of the voltage
fluctuation on light intensity [2].
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1.3 PQ Disturbances Analysis Tools
Simulation Software: DIgSILENT (https://www.digsilent.de/en/)
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
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1: PQ Disturbances and Analysis
www.electrotek.com
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
https://www.electrotek.com/pqview-4/
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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ETAP
1: PQ Disturbances and Analysis
https://etap.com/
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1: PQ Disturbances and Analysis
Conclusions
• Various PQ disturbances can occur in a power system,
resulting in various consequences.
• Different PQ disturbances possess different
characteristics. Therefore, proper analysis methods and
mitigation techniques are required accordingly.
• Waveforms and RMS plots are typical presentation for
PQ disturbances.
• Voltage sag, harmonics, voltage imbalance and voltage
fluctuation are main PQ disturbances and can be
commonly found in a power system.
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
References
[1] IEC 61000-4-30 (2015): Testing and Measurement
Techniques – Power Quality Measurement Methods
[2] IEEE 1159 (2019): IEEE Recommended Practice for
Monitoring Electric Power Quality
[3] IEC 61000-2-5 (2011): Environment – Description and
Classification of Electromagnetic Environments
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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1: PQ Disturbances and Analysis
Questions & Discussions
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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Power Quality Problems,
Power System Protection and Solution:
Theory and Case Studies
2. PQ Monitoring for Troubleshooting
6-8 Sept 2022 @ The Sukosol Hotel
Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
Power System Research Lab
Dept. Electrical Engineering, Chulalongkorn University
Email: Thavatchai.t@chula.ac.th
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2. PQ Monitoring for Troubleshooting
Section Overview
• What are PQ problems?
• Why need PQ monitoring? Location?
• Measurement instruments
• How to interpret measurement results?
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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2. PQ Monitoring for Troubleshooting
2.1 PQ Problems and PQ Monitoring
PQ Problems
Consequences from PQ disturbances that can seriously affect
a power system and result in failure or misoperation of loads.
For example,
Voltage sags can cause sensitive equipment to shut down.
Harmonic resonances can cause high harmonic voltages and
currents in a power system damaging electrical equipment.
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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2. PQ Monitoring for Troubleshooting
2.1 PQ Problems and PQ Monitoring
PQ Monitoring
In order to understand characteristics of PQ problems, PQ
monitoring has to be carried out. Then proper mitigation
methods can be implemented accordingly.
Monitoring types [1]
1. Diagnostic monitoring to solve shutdown problems with
sensitive equipment
2. Evaluative or predictive monitoring to characterize the
existing level of PQ with collection of several voltage and
current parameters
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2. PQ Monitoring for Troubleshooting
2.1 PQ Problems and PQ Monitoring
PQ Problem
Evaluation [2]
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
Objectives [1, 3]:
1. Troubleshooting: To diagnose incompatibilities between the
electric power source and the load
2. PQ evaluation: To evaluate the electrical environment at a
particular location to refine modeling techniques or to
develop a power quality baseline
3. Planning the connection of new equipment: To predict
future performance of load equipment or power quality
mitigating devices
* Clearly define the objectives in any monitoring project *
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
Measurement chain [3]:
Electrical quantity may be either directly accessible, as is
generally the case in LV systems, or accessible via measurement
transducers such as voltage and current transformers.
Measurement chain
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
Classes of measurement [3]
1. Class A (A stands for “Advanced”)
This class is used where precise measurements are necessary.
Two different instruments complying with Class A, when
measuring the same signals, will produce matching results
within specified uncertainty for that parameter.
2. Class S (S stands for “Surveys”)
This class is used for statistical applications such as surveys or
PQ assessment, with a limited subset of parameters. Class S
processing requirements are much lower than class A.
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2.2 Monitoring Objectives and Locations
The choice of locations to install PQ monitors will be
dependent upon the objective of the survey [1].
• If the objective is to diagnose an equipment performance
problem, then the monitor should be places as close to the
load as possible.
• If the objective is to investigate the overall quality of a
facility, then the monitor should be placed on the
secondary side of the main service entrance transformer.
• If harmonics are concern, the monitor should be placed at
the affected equipment, capacitor banks or filter locations.
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
Suggested Monitoring Locations [1]
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
Suggested Monitoring Locations [1]
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
The choice of monitoring thresholds and period [3]
• Thresholds are used for detecting events, e.g. dips, swells.
For statistical analysis of quasi-steady-state parameters
such as harmonics, unbalance and flicker, continuous
recording without thresholds is needed.
• Choice of monitoring thresholds may be determined by the
PQ indices against which the results are to be compared.
• Thresholds should be as tight as feasible. Data will be
missed to loose thresholds. Points to consider are trigger
level and reporting method by the PQ monitors.
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2.2 Monitoring Objectives and Locations
The choice of monitoring thresholds and period [3]
• Monitoring period will be determined by the reasons for
performing the PQ survey. The appropriate time to monitor
is the time corresponding with the PQ symptom that is
being investigated [1].
• Event measurements such as dips and swells generally
require longer measurement periods to capture enough
events to provide meaningful statistics (months).
• Harmonics and other steady-state measurements,
meaningful information may be captured in relatively short
periods of time (minimum of one week)
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2. PQ Monitoring for Troubleshooting
2.2 Monitoring Objectives and Locations
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2.2 Monitoring Objectives and Locations
Statistical assessment
• 95th percentile limits
Example, EN 50160 (LV supply characteristics)
1. Supply voltage variation: During each period of one week
95% of the 10 min mean rms value of the supply voltage
shall be within the range of Un+/-10%
2. Flicker severity: In any period of one week, Plt ≤ 1 for 95%
of the time
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
PQ monitors are a compromise between cost, portability and
completeness [1]. The type of PQ monitors is based on the
need to monitor. Types of PQ meters
1. Portable/handheld monitors: To troubleshoot problems
2. Permanently installed units: To monitor longer-term system
performance and reliability and provide data and alarms
when PQ problems occur
Voltage and current are the primary two measurements for PQ
phenomena. Most instrument use analog-to-digital converters
to sample and store voltage and current waveforms.
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Example of PQ
monitors [4]
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Example of PQ monitor
technical specifications [5]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Example of PQ monitor
standard compliance [5]
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2.3 Measurement Instruments
Example of PQ monitor measurement specifications [5]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Example of PQ monitor measurement specifications [5]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Example of PQ monitor measurement specifications [6]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Decisions on connecting the measurement equipment will be
heavily influenced by the reason for the survey [3]. These
decisions include:
• Single-phase vs three-phase measurement
• Line-to-line vs line-to-neutral or line-to-ground connection
• High side vs low side measurement near transformers
Single-phase measurements can often be made for harmonics
and flicker. Voltage dips and swells, it is necessary to monitor
all phases powering the affected equipment.
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Connections
of PQ meters [7]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Connections
of PQ meters [7]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Current
Probes [7]
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Transducers [3]
1. The most common voltage transducer is the voltage
transformer (VT) or potential transformer (PT). Two types
used: those used by protective relay circuits (correct
response for overvoltages) and those used by metering
circuits (wrong response for overvoltages due to
saturation).
2. The most common type of current transducer is the
current transformer (CT). The correct secondary should be
selected for the intended measurement. Open circuits on
the secondary winding can give rise to dangerously high
voltages.
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2. PQ Monitoring for Troubleshooting
2.3 Measurement Instruments
Recommendations for transducers [1]
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2.3 Measurement Instruments
Frequency response of transducers [3]
1. Voltage transducer: Transformer-type electromagnetic
VTs have frequency and transient responses suitable up to
typically 1 kHz.
2. Current transducer: The frequency response of CT varies
according to the uncertainty class, type, turn ratio, core
material and cross section and the secondary circuit load.
Usually, the cut-off frequency ranges from 1 kHz to a few
kHz, and the phase response degrades as the cut-off
frequency is approached.
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2.4 Interpreting PQ Monitoring Results
Troubleshooting [1]
• Many problems are solved by carefully examining the load,
others by verifying correct wiring and grounding practices.
• No single practice will handle every problem.
• Investigator should have enough knowledge and skill to
produce a solution from the available data.
• Interpreting a power monitor’s output is perhaps the most
critical part of the process of power monitoring.
• Plant personnel should keep a log of equipment
malfunctions, time of occurrence, equipment affected.
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2.4 Interpreting PQ Monitoring Results
Measurement validation [1]
• Examine the power monitor configuration and confirm
that it matches the power system monitored
• The interval generally should be at least one power period
(repeated pattern).
• Recorded data are reasonable based on the circuit
configuration, monitor capabilities, and monitor
connection method.
• Check magnitudes are reasonable and time stamps are
within the monitoring window. Use power values to check
connection type, polarities of CTs
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2.4 Interpreting PQ Monitoring Results
Interpreting data [1]
• Chronological overview of what occurred during the
monitoring interval can be reviewed and compared to load
cycles, failure logs and result from personnel interviews.
• Patterns in time or disturbance characteristics may show
the true source of the problem.
• Critical data extraction need to be examined more closely
in order to find a proper solution.
• Take the critical data and combine them into events
• Be careful when looking at time stamp to determine what
goes with what
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2. PQ Monitoring for Troubleshooting
2.4 Interpreting PQ Monitoring Results
Problem
analysis
[1]
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2. PQ Monitoring for Troubleshooting
2.4 Interpreting PQ Monitoring Results
Problem
analysis
[1]
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2. PQ Monitoring for Troubleshooting
2.4 Interpreting PQ Monitoring Results
Interpreting data [1]
• Signature analysis: Seeing a certain signature
(characteristic graphical representation of PQ
disturbances) can identify the presence of that load.
• Waveshape analysis: Normal steady-state waveshape
should be examined to see what might happening when
there are no disturbances.
• Sag analysis: RMS plot should be used for further analysis
to provide sag magnitude and duration. Voltage sag with a
corresponding increase current is known to have a cause
“downstream” of the monitoring location.
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2.4 Interpreting PQ Monitoring Results
Interpreting data [1]
• Harmonic analysis:
1. Waveform distortion is expected when harmonics are present.
2. High neutral current in a three-phase wye-connected system
can be a result of high 3rd harmonics.
3. Single-phase power conversion devices will typically produce
high 3rd harmonic current distortion.
4. If there are significant even-order harmonics, then signal’s
positive half cycle and negative half cycle are not symmetrical.
5. For p-pulse conveter, harmonics at pk  1, where k is an integer
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2.4 Interpreting PQ Monitoring Results
Interpreting data [1]
• Pattern recognition: Few disturbance patterns occur
naturally. Some examples are
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2.4 Interpreting PQ Monitoring Results
Verifying data [1]
• After the cause of PQ disturbances is identified. A possible
solution is suggested.
• Double-check the solution to see if it is, indeed, the right
answer for the problem. Computer simulation tools can be
used.
• If the answer is “no” to 1) Is the failing equipment now
operating correctly? and 2) Is there a reduction or
elimination of the disturbance in question?, then further
investigation is warranted.
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2. PQ Monitoring for Troubleshooting
2.4 Interpreting PQ Monitoring Results
Verifying data [1]
• Many times solving one problem simply allows the next
problem to surface.
• Post-monitoring should be used to verify the validity of
solution or the success of the solution.
• Post-monitoring helps to determine whether any other
concerns have arisen because of the implementation of a
solution.
• Economics of possible solutions should be evaluated.
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2. PQ Monitoring for Troubleshooting
Conclusions
• In order to understand characteristics of PQ problems, PQ
monitoring is required.
• Monitoring objectives are very crucial to determine choice of
locations, monitoring period, types of monitors and monitor
connections.
• Knowledge, skills and various techniques can be used to solve
PQ problems.
• No single practice will handle every problem. Solving one
problem simply allows the next problem to arise.
• Post-monitoring should be carried out to verify the validity of
solution. Computer simulation tools are also useful.
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2. PQ Monitoring for Troubleshooting
References
[1] IEEE 1159 (2019): IEEE Recommended Practice for
Monitoring Electric Power Quality
[2] R. C. Dugan, M. F. McGranaghan, S. Santoso, and H. W. Beaty,
Electrical Power Systems Quality, 2ed. McGraw Hill, New York,
2004
[3] IEC 61000-4-30 (2015): Testing and Measurement
Techniques – Power Quality Measurement Methods
[4] http://en-us.fluke.com/products/power-quality-analyzers/
[5] PM180, SATEC
[6] ION 7650, PowerLogic, Schneider Electric
[7] PV440, PX5, Dranetz
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Questions & Discussions
© Assoc. Prof. Thavatchai Tayjasanant, Ph.D.