CEE208: Power Quality

Assignment-1

Submitted By: ATIF MONIR (2K21/CEEE/10)

Q1. Explain the key factors used in the classification of power quality issues.
How do these factors impact electrical systems?

Introduction:

Power quality (PQ) refers to the degree to which the electrical power supplied to consumers is
free of disturbances and conforms to a pure sinusoidal waveform at the rated frequency and
voltage. Power quality issues are classified based on various measurable and observable
parameters that define the nature, duration, and source of disturbances.

Understanding these key classification factors is essential for diagnosing problems, improving
system reliability, and protecting sensitive equipment.

Key Factors Used in Classification of Power Quality Issues:

1. Duration of Disturbance

Power quality events are categorized by how long they persist:

 Transient (sub-cycle):
Duration < 1 cycle (typically < 20 ms at 50 Hz).
Examples: Lightning surges, switching spikes.
 Short Duration:
o Sags (voltage dip): 0.5 cycle to 1 minute.
o Swells (voltage rise): 0.5 cycle to 1 minute.
o Interruptions: Complete voltage loss (up to 1 minute).
 Long Duration:
 o Undervoltages or overvoltages lasting more than 1 minute due to system faults or
transformer tap changes.

Impact: Longer duration events affect process continuity, especially in automation, motors, and
control systems.

2. Magnitude (Amplitude) of Disturbance

Refers to how much the voltage or current deviates from the nominal level.

Type Typical Range

Voltage Sag 10% to 90% of nominal
Voltage Swell 110% to 180% of nominal
Harmonic Distortion Up to 10% or more depending on load

Impact: High magnitude deviations can lead to overheating, tripping of protective devices, data
corruption in IT systems, or even equipment damage.

3. Waveform Distortion

Ideally, the voltage waveform is purely sinusoidal. In practice, nonlinear loads distort the
waveform. Types of distortion:

 Harmonics: Integer multiples of the fundamental frequency (e.g., 150 Hz, 250 Hz).
 Interharmonics: Non-integer multiples (e.g., 175 Hz).
 Notching: Periodic voltage drop due to phase-angle controlled rectifiers.
 Noise: Superimposed high-frequency disturbances from EMI/RFI.

Impact: Harmonic distortion reduces equipment life, causes overheating, and interferes with
communication systems.

4. Frequency Deviation

In power systems, frequency should remain constant (50 Hz or 60 Hz). Deviations indicate
imbalance between generation and load.

 Causes: Generator trip, sudden load changes.

 Effect: Frequency-sensitive devices (e.g., synchronous clocks, UPS systems) may
malfunction.
Impact: Can lead to misoperation of frequency-sensitive controls and instability in power grids.

5. Symmetry or Balance (in Three-Phase Systems)

In a balanced system:

 All phase voltages and currents are equal in magnitude.

 Phase angles are 120° apart.

Unbalance Types:

 Voltage Unbalance: Occurs due to asymmetrical loads or transformer faults.

 Current Unbalance: Caused by unequal distribution of single-phase loads.

Impact: Creates negative and zero-sequence components, increasing heating and losses in
motors and transformers.

6. Repetition Rate or Pattern

Disturbances may occur:

 Continuously: Harmonics due to continuous nonlinear loads.

 Periodically: Voltage sags during routine motor starts.
 Randomly: Transients from lightning or switching.

Impact: Repetitive events like flicker can lead to long-term equipment stress and human
discomfort, while random events require rapid protection schemes.

Summary Table:

Classification Factor Examples Major Impact

Duration Sag, Swell, Interruption Process disruption, equipment reset
Magnitude Overvoltage, Undervoltage Tripping, overheating, failure
Waveform Distortion Harmonics, Noise Heat, malfunction, EMI
Frequency Grid imbalance Misoperation, instability
Symmetry/Unbalance Voltage/current imbalance Motor torque pulsation, transformer heating
Repetition Flicker, transients Visible discomfort, insulation stress
Conclusion:

The classification of power quality issues using parameters like duration, magnitude, distortion,
symmetry, and repetition provides a structured framework for identifying and addressing
electrical disturbances. Each factor not only affects the technical operation of electrical
equipment but also has economic and operational consequences, especially in sensitive or
automated industries. Proper classification enables accurate diagnosis, targeted mitigation, and
compliance with standards such as IEEE 1159 and IEC 61000.

Q2. Define electric power quality. What are the primary parameters used to
characterize electric power quality?

Definition of Electric Power Quality:

Electric Power Quality (PQ) refers to the degree to which the characteristics of the electrical
supply conform to established standards and are suitable for the reliable and efficient operation
of connected loads.

A system is said to have good power quality if the voltage, current, and frequency remain close
to their rated values at all times and under all load conditions. Any deviation from the ideal
sinusoidal waveform can cause disturbances that affect the performance and lifespan of electrical
equipment.

According to IEEE 100-1992, power quality is:

“The concept of powering and grounding sensitive electronic equipment in a manner that is
suitable for the operation of that equipment.”

Importance of Electric Power Quality:

 Ensures uninterrupted and efficient operation of devices.

 Minimizes losses, overheating, and maintenance costs.
 Prevents malfunction or damage of sensitive equipment (e.g., computers, medical
devices).
 Improves energy efficiency and compliance with regulatory standards.

Primary Parameters Used to Characterize Power Quality:

1. Voltage Magnitude and Stability

 Ideally, the voltage should remain within ±5% of its nominal value.
 Voltage deviations include:
o Undervoltage (continuous low voltage)
o Overvoltage (continuous high voltage)
o Sags/Dips (short-term drops)
o Swells (short-term rises)

Impact: Fluctuating voltage levels affect motor performance, reduce efficiency, and cause
nuisance tripping.

2. Frequency

 Standard frequency: 50 Hz (India) or 60 Hz (US).

 Must remain within ±0.5 Hz of nominal in most grids.

Impact: Frequency deviation indicates generation-load imbalance and can affect synchronous
machines and digital equipment.

3. Harmonics

 Harmonics are voltage or current waveforms at multiples of the fundamental frequency.

 Total Harmonic Distortion (THD) is used to quantify harmonics.

Impact: Causes overheating in transformers/motors, misoperation of protection devices, and loss

in power system efficiency.

4. Waveform Shape

 Ideally sinusoidal.
 Distorted waveforms can be caused by:
o Non-linear loads
o Switching operations
o Electronic converters
Impact: Poor waveform shape leads to energy losses and equipment malfunction.

5. Voltage Fluctuations and Flicker

 Small, repetitive voltage changes can cause flickering lights and discomfort.
 Measured using short-term (Pst) and long-term (Plt) indices.

Impact: Affects human visual perception and can be irritating in offices or medical
environments.

6. Unbalance in Three-phase Systems

 In an ideal three-phase system:

o Voltage magnitudes are equal.
o Phase angles are 120° apart.

Unbalanced Conditions: Result from single-phase loads or asymmetrical faults.

Impact: Increases losses, causes motor vibration, and reduces efficiency.

7. Transients and Surges

 Sudden, short-duration increases or drops in voltage or current.

 Types:
o Impulsive transients (e.g., lightning)
o Oscillatory transients (e.g., capacitor switching)

Impact: Can damage sensitive electronics or reset control systems.

Summary Table of Key Parameters:

Parameter Description Acceptable Range / Metric

Voltage Steady-state and variations ±5% of nominal
Frequency Oscillation rate of supply 49.5–50.5 Hz (India)
Harmonics (THD) Non-sinusoidal waveform distortion ≤ 5% (IEEE 519)
Waveform Shape Sinusoidal ideal No distortion or flattening
Flicker Rapid voltage changes Pst < 1.0
 Parameter Description Acceptable Range / Metric
Unbalance Unequal voltages/currents in 3 phases ≤ 2% for voltage, ≤ 5% for current
Transients Very fast voltage or current spikes Suppressed by surge protection

Conclusion:

Electric power quality is a multi-faceted concept that encompasses voltage, current, frequency,
and waveform integrity. By monitoring and controlling key parameters like voltage deviation,
frequency fluctuation, harmonics, and unbalance, we ensure stable and reliable power delivery.
This is crucial not only for industrial equipment and processes but also for residential and
commercial applications where power-sensitive devices are increasingly common.

Q3. What are power acceptability curves? How are they used to evaluate the
severity of power quality disturbances?

Definition of Power Acceptability Curves:

Power acceptability curves are graphical tools that define the tolerable boundaries of voltage
disturbances—such as sags, swells, and interruptions—within which electrical equipment can
operate without malfunction or damage.

They relate magnitude (depth) and duration of a voltage event to determine whether that event
is acceptable for a particular class of equipment. These curves are essential in designing,
analyzing, and monitoring power quality performance for sensitive electronic loads.

Common Types of Power Acceptability Curves:

1. ITIC Curve (formerly CBEMA Curve):

 Developed by the Information Technology Industry Council (ITIC).

 Widely used for evaluating tolerance of IT equipment.
 X-axis: Duration of voltage disturbance (in cycles or seconds).
 Y-axis: % of nominal voltage (sag or swell).

Zones:

 Safe Operating Region: Equipment operates normally.

 No Damage but Malfunction Zone: Equipment may trip/reset.
  Prohibited Zone: Equipment likely to be damaged.

2. SEMI F47 Curve:

 Developed by Semiconductor Equipment and Materials International.

 Designed for semiconductor fabrication tools, which are highly sensitive to sags.
 Tighter requirements than ITIC; for example, it requires:
o Equipment must withstand 50% voltage sag for 200 ms
o 70% voltage for 500 ms
o 80% for 1 second

3. ANSI C84.1 and EN 50160:

 Define long-duration voltage ranges for utility delivery points (e.g., ±5% steady-state
variation).
 Used more for compliance at service entrances than for short-term events.

How Are These Curves Used?

1. Evaluating Voltage Sags and Swells:

 When a sag event occurs, its depth (e.g., 80% of nominal) and duration (e.g., 200 ms)
are plotted on the curve.
 If the point lies within the acceptable region, the equipment is not expected to
malfunction.
 If it falls in a prohibited region, then the event is too severe.

2. Equipment Design:

 Equipment manufacturers use these curves to design devices that tolerate standard
disturbances.
 Allows for standardized performance expectations across industries.

3. Power Quality Monitoring:

  Utilities and facility managers compare actual events against these curves.
 Helps in:
o Identifying vulnerable equipment
o Classifying the severity of events
o Planning mitigation (like DVRs, UPS)

Example:

A voltage sag drops the supply to 70% for 100 ms.

Plotting this on the ITIC curve shows it lies in the acceptable operating region, meaning:

 No trip/reset expected
 Equipment should continue to function

But the same event may violate the SEMI F47 standard for semiconductor tools, triggering
downtime.

Illustration (described):

📉 Graph with x-axis = duration (ms or cycles), y-axis = % voltage

 A shaded “acceptable region” defines boundaries for sag/swell events.

 Events falling outside this region are flagged as problematic.

(Actual diagrams would be included in reports or slide presentations.)

Conclusion:

Power acceptability curves provide a visual and analytical method to assess whether a power
disturbance will impact equipment operation. They serve as industry benchmarks for evaluating
power quality events and planning mitigation strategies. Widely accepted standards like the
ITIC and SEMI F47 curves allow engineers and utilities to compare disturbances against pre-
defined tolerance thresholds and take corrective action where needed.

Q4. Differentiate between steady-state and transient power quality disturbances.

Provide examples of each.

1. Steady-State Disturbances:
  These are continuous or long-duration deviations in voltage, current, or frequency.
 The disturbance remains constant or varies slowly over time.
 Typically predictable and easier to monitor and mitigate.

Examples:

 Harmonics from non-linear loads

 Voltage unbalance
 Flicker due to arc furnaces
 Overvoltage/undervoltage from load fluctuations

Impact: Can cause overheating of equipment, misoperation of sensitive devices, and decreased
efficiency.

2. Transient Disturbances:

 These are sudden, short-duration disturbances in the power system.

 Caused by fast changes such as switching, faults, or lightning strikes.
 Duration is usually less than one cycle (sub-cycle).

Types:

 Impulsive transients: Unidirectional (e.g., lightning strike)

 Oscillatory transients: Ringing effect (e.g., capacitor switching)

Examples:

 Switching of inductive loads

 Motor startup transients
 Lightning-induced surges
 Transformer energization

Impact: Can damage sensitive electronics, disrupt communication systems, or trip protective
devices.

Comparison Table:

Aspect Steady-State Disturbance Transient Disturbance

Duration Long or continuous Short (microseconds to ms)
Nature Persistent Sudden and temporary
Examples Harmonics, flicker Lightning, switching surges
Monitoring Easier with standard meters Needs high-speed recorders
 Aspect Steady-State Disturbance Transient Disturbance
Impact Cumulative (heat, stress) Immediate (damage, resets)

Q5. How does a poor load power factor affect power systems? Discuss possible
solutions for improving load power factor.

Impact of Poor Power Factor:

1. Higher Current Demand:

o More current is needed to deliver the same real power.
o Increases conductor size and losses.
2. Voltage Drops:
o Poor PF leads to excessive voltage drops across the system.
3. Overloading of Equipment:
o Transformers, generators, and cables experience increased thermal loading.
4. Reduced System Efficiency:
o Increases I²R losses and operational costs.
5. Penalties from Utility Providers:
o Utilities impose charges for PF below 0.9 or 0.95.

Solutions to Improve Power Factor:

1. Capacitor Banks:
o Static correction at load points.
o Supplies leading VARs to counter inductive lagging PF.
2. Synchronous Condensers:
o Overexcited synchronous motors running without load.
3. Active Power Factor Correction (APFC):
o Electronic systems that adjust PF dynamically.
4. Phase Advancers:
o Used with induction motors for local PF correction.

Example:

If a 100 kW motor operates at 0.75 PF lagging, its apparent power:

=> Smaller cable size, reduced losses, and lower electricity bills.

Q6. Explain how non-linear and unbalanced loads contribute to power quality
problems. What mitigation techniques can be applied?

Non-Linear Loads:

 Draw non-sinusoidal currents even if voltage is sinusoidal.

 Introduce harmonics into the system.

Examples: Computers, UPS, VFDs, rectifiers, arc furnaces.

Problems Caused:

 Transformer and motor overheating

 Resonance issues
 False tripping of relays

Unbalanced Loads:

 Occurs when the three-phase system has unequal phase loads.

 Results in negative and zero-sequence components.

Problems Caused:

 Voltage unbalance
 Torque pulsations in motors
 Increased heating and mechanical stress

Mitigation Techniques:
Problem Type Mitigation
Harmonics Passive filters, Active filters, K-rated transformers
Unbalance Load redistribution, Phase balancing transformers
Reactive Power Capacitor banks, APFC panels

Q7. What is "notching" in load voltage? Describe its causes and effects on power
systems.

Definition:

Notching is a type of waveform distortion that appears as periodic voltage dips (notches) during
current commutation in power electronic converters.

Causes:

 Common in systems with:

o Thyristor-based converters
o Phase-controlled rectifiers
 Occurs when current transfers between power devices cause short durations of low or
zero voltage.

Effects:

 Harmonic generation
 Interference with communication circuits
 Damage to sensitive control equipment
 Overheating in neutral conductors

Mitigation Techniques:

 Isolation transformers
 Smoothing reactors
 Replacing older thyristor drives with IGBT-based converters
 Line filters (notch filters)
Q8. Discuss the phenomenon of voltage sags and swells. What are the typical
causes, and how can they be mitigated?

Voltage Sag (Dip):

 Short-duration reduction in RMS voltage (10–90%).

 Duration: 0.5 cycle to 1 minute.

Causes:

 Large motor starting

 Faults in distribution network
 Transformer energizing

Impacts:

 Motor stalling
 PLC/control equipment reset
 Data loss in sensitive electronics

Voltage Swell:

 Short-duration increase in RMS voltage (110–180%).

Causes:

 Sudden disconnection of large load

 Switching off reactive components
 Utility-side voltage regulation failure

Mitigation Techniques:

 Dynamic Voltage Restorer (DVR)

 Uninterruptible Power Supply (UPS)
 Voltage regulators
 Fault-tolerant controls

Q9. What are power quality indices, and why are they important? Explain the
distortion index and C-message index.
Power Quality Indices:

These are numerical values used to quantify power quality disturbances.

Importance:

 Enables monitoring and benchmarking

 Facilitates compliance with IEEE/IEC standards
 Helps identify and isolate disturbances

Key Indices:

Index Description
THD (Total Harmonic Distortion) Measures total harmonic content
Flicker Index (Pst, Plt) Measures lighting flicker sensitivity
Sag Index (ITIC/CBEMA) Evaluates tolerance to sags
C-message Index Measures noise affecting communication circuits

Distortion Index (THD):

A THD
above 5% is generally considered poor for voltage.

C-message Index:

 Weighted measurement of noise based on human speech sensitivity.

 Used in telecom circuits to assess interference.

Q10. Describe the role of IEEE guides and recommended practices in

maintaining power quality. How do these standards help in diagnosing and
mitigating power quality issues?

Role of IEEE Standards:

IEEE provides frameworks and guidelines for monitoring, analyzing, and mitigating power
quality problems. These are used globally by utilities and industries.

Key IEEE Standards:

Standard Focus Area

IEEE 519 Harmonic limits at Point of Common Coupling
IEEE 1159 Recommended practice for monitoring PQ
IEEE 1100 Powering sensitive electronic equipment
IEEE C62.41 Surge protection guidelines

Functions of These Standards:

1. Measurement Protocols: Defines how disturbances like voltage sags, swells, harmonics
are measured.
2. Thresholds and Limits: Specifies acceptable ranges (e.g., THD < 5%, flicker < 1.0).
3. Mitigation Techniques: Provides recommended filtering, grounding, and compensation
strategies.
4. Diagnostics and Reporting: Ensures uniform documentation and event classification.

Conclusion:

IEEE standards serve as a universal language for engineers and grid operators to assess,
diagnose, and rectify power quality issues efficiently. Adhering to these ensures compliance,
safety, and long-term system reliability.

Q11. A three-phase system experiences voltage sags of 20% for 10 cycles every 5
minutes. Calculate the power quality index for this system and determine
whether it meets IEEE standards.

Step 1: Understanding the Event

 Voltage sag magnitude = 20% sag → Voltage drops to 80% of nominal.

 Duration of each event = 10 cycles
 Frequency of event = Once every 5 minutes
Step 2: Duration in Time

⇒ 10 cycles = 10×20 ms=200 ms

At 50 Hz, 1 cycle = 20 ms

So, each sag lasts for 200 milliseconds.

Step 3: Referencing the ITIC (CBEMA) Curve

The ITIC (Information Technology Industry Council) curve provides tolerable boundaries for
voltage sags and swells for sensitive electronic equipment.

For a voltage drop to 80% of nominal:

 The ITIC curve permits a sag lasting up to 10 seconds.

Since this sag is only 200 ms, it falls within acceptable limits.

Step 4: Power Quality Index (PQI)

Let’s define a simplified PQI metric for sag severity:

Using:

 Sag depth = 20% (i.e., voltage drop from 100% to 80%)

 Duration = 200 ms
 Allowed duration at 80% = 10 seconds = 10,000 ms

Interpretation:

PQI < 100% ⇒ within limits

PQI > 100% ⇒ violation of standard


Conclusion:
The PQI is 40%, which is well within the IEEE/ITIC tolerance limits. Therefore, this system
meets the IEEE power quality standards for voltage sags.

Q12. A system has a load drawing a fundamental current of 50 A but also

produces 10 A of third-harmonic current. Calculate the Total Harmonic
Distortion (THD) and analyze its impact on power quality.

Given:

 Fundamental current I1=50 A

 3rd harmonic current I3=10 A
 No other harmonics specified

Step 1: THD Formula

Step 2: Interpretation of THD

According to IEEE 519:

 For current distortion at PCC (Point of Common Coupling) on systems <69 kV, THD
should be ≤ 5%.
 Here, THD = 20%, which exceeds the acceptable limit.

Impact on Power Quality:

 1. Transformer Heating:
o Harmonics increase eddy and hysteresis losses.
2. Neutral Conductor Overload:
o 3rd harmonics add arithmetically in neutrals.
3. Reduced Equipment Life:
o Continuous exposure to harmonics damages insulation.
4. Sensitive Device Malfunction:
o Distorted currents affect electronics and protective relays.

Conclusion:

The calculated THD of 20% is excessive and violates IEEE 519 guidelines.
To improve power quality:

 Install harmonic filters

 Use K-rated transformers
 Apply 12-pulse rectifiers instead of 6-pulse