SECTION A – Short Answers (2 Marks Each)
(a) Meaning of power quality disturbances
Power quality disturbances refer to any deviation in voltage, current, or frequency from ideal sinusoidal conditions that can cause malfunction or damage to electrical equipment.
(b) Causes of voltage sags and swells
Voltage sags are caused by short circuits, motor starting, and heavy load switching.
Voltage swells occur due to sudden load removal, capacitor switching, or faulty voltage regulation.
(c) Types of sag mitigation devices
Two sag mitigation devices are Dynamic Voltage Restorer (DVR) and STATCOM.
(d) Importance of estimating sag performance
Sag performance estimation helps in identifying system vulnerability, improving equipment immunity, planning mitigation devices, and ensuring reliable power supply.
(e) Types of transient overvoltages
Transient overvoltages are classified as impulsive transients (lightning) and oscillatory transients (capacitor switching).
(f) Effect of capacitor switching transients on network
Capacitor switching causes high inrush currents and oscillatory overvoltages, leading to insulation stress, equipment damage, and harmonic amplification.
(g) Need of FACTS controllers in power system
FACTS controllers improve power transfer capability, enhance voltage stability, reduce losses, control power flow, and improve overall system reliability.
(h) V–I characteristics of SVC
SVC exhibits a variable reactive power characteristic where reactive current varies linearly with voltage within control limits, providing voltage regulation.
(i) Harmonic sources from industrial loads
Industrial harmonic sources include rectifiers, variable frequency drives (VFDs), arc furnaces, UPS systems, and power electronic converters.
(j) Harmonic index and its significance
Harmonic index quantifies the level of harmonic distortion in a system. It is significant for assessing power quality, equipment heating, and compliance with standards.
SECTION B – Long Answers (10 Marks Each)
(a) Power Quality Evaluation Procedure & General Classes of PQ Problems
Power quality evaluation involves monitoring voltage, current, harmonics, transients, and interruptions using PQ analyzers.
General classes of power quality problems include:
Voltage sags and swells
Interruptions
Transients
Harmonic distortion
Voltage imbalance
Flicker
Proper evaluation helps in identifying root causes and selecting mitigation solutions.
(b) Information Technology Industry Council (ITIC) curve
The ITIC curve defines voltage tolerance limits for IT equipment.
It categorizes events into:
Normal operation zone
Prohibited region (equipment damage)
No-damage but malfunction region
The curve helps assess whether voltage disturbances affect sensitive electronic equipment.
(c) Lightning, surge arrestors and surge suppressors
Lightning is a high-energy electrical discharge between clouds or between cloud and earth.
Surge arrestors divert high-energy surges to ground, protecting insulation.
Surge suppressors limit transient overvoltages at equipment terminals using nonlinear elements like MOVs.
(d) Types of FACTS controllers
FACTS controllers are classified as:
Series controllers (TCSC, SSSC)
Shunt controllers (SVC, STATCOM)
Series-Series controllers (IPFC)
Series-Shunt controllers (UPFC)
They regulate voltage, impedance, and phase angle to improve power system performance.
(e) Harmonic indices used in power systems
Two important harmonic indices are:
Total Harmonic Distortion (THD) – measures overall harmonic content
Individual Harmonic Distortion (IHD) – measures specific harmonic order
These indices help in evaluating waveform distortion and equipment stress.
SECTION C – Long Answers (10 Marks Each)
3(a) Short-duration voltage variations and mitigation
Short-duration voltage variations include sags, swells, and interruptions, lasting from half a cycle to one minute.
Mitigation techniques include DVR, STATCOM, UPS, and static transfer switches.
Example: DVR injects compensating voltage during sag conditions to maintain load voltage.
3(b) Waveform distortion and its categories
Waveform distortion refers to deviation from pure sinusoidal waveform.
Categories include:
Harmonic distortion
Interharmonics
DC offset
Noise
Notching
These distortions degrade equipment performance and increase losses.
4(a) Static Transfer Switch (STS) and sag mitigation
STS rapidly transfers load between two independent power sources during voltage sag.
It minimizes sag severity by ensuring uninterrupted supply to critical loads such as data centers and hospitals.
4(b) Sag mitigation techniques
(i) Hybrid UPQC with energy storage: Combines series and shunt compensation with storage to mitigate sags and harmonics.
(ii) Flywheel energy storage: Stores kinetic energy and supplies power during short-term voltage sags.
5(a) Lightning current range and mechanism
Lightning stroke current ranges from 10 kA to 200 kA.
Mechanism involves charge separation in clouds, leader formation, and return stroke causing high-energy discharge.
5(b) Principles of overvoltage protection of load equipment
Protection principles include insulation coordination, surge diversion, grounding, shielding, and use of protective devices like arrestors and suppressors.
6(a) Operation of DSTATCOM and comparison with SVC
DSTATCOM is a shunt-connected FACTS device that injects reactive current using a voltage source converter.
Compared to SVC, DSTATCOM offers faster response, better performance at low voltages, and smaller size.
6(b) Static VAR system
A static VAR system provides dynamic reactive power compensation using thyristor-controlled devices.
Advantages include improved voltage stability and power factor.
Applications include transmission lines, industrial loads, and renewable integration.
7(a) Harmonic filter & benefits of active harmonic filter
A harmonic filter reduces harmonic currents in power systems.
Active harmonic filters dynamically inject compensating currents, offering adaptability, reduced resonance, and improved power quality.
7(b) Harmonic terms
(i) Individual & Total Harmonic Distortion:
IHD measures single harmonic component, THD measures combined effect.
(ii) Zig-zag transformer:
Used for harmonic mitigation and grounding, especially for reducing triplen harmonics.
